perl boolean assignment

• Operator Precedence and Associativity
• Terms and List Operators (Leftward)
• The Arrow Operator
• Auto-increment and Auto-decrement
• Exponentiation
• Symbolic Unary Operators
• Binding Operators
• Multiplicative Operators
• Additive Operators
• Shift Operators
• Named Unary Operators
• Relational Operators
• Equality Operators
• Class Instance Operator
• Smartmatching of Objects
• Bitwise And
• Bitwise Or and Exclusive Or
• C-style Logical And
• C-style Logical Or
• C-style Logical Xor
• Logical Defined-Or
• Range Operators
• Conditional Operator
• Assignment Operators
• Comma Operator
• List Operators (Rightward)
• Logical Not
• Logical And
• Logical or and Exclusive Or
• C Operators Missing From Perl
• Quote and Quote-like Operators
• Regexp Quote-Like Operators
• Quote-Like Operators
• Gory details of parsing quoted constructs
• I/O Operators
• Constant Folding
• Bitwise String Operators
• Integer Arithmetic
• Floating-point Arithmetic
• Bigger Numbers
• List of Extra Paired Delimiters

perlop - Perl expressions: operators, precedence, string literals

# DESCRIPTION

In Perl, the operator determines what operation is performed, independent of the type of the operands. For example $x + $y is always a numeric addition, and if $x or $y do not contain numbers, an attempt is made to convert them to numbers first.

This is in contrast to many other dynamic languages, where the operation is determined by the type of the first argument. It also means that Perl has two versions of some operators, one for numeric and one for string comparison. For example $x == $y compares two numbers for equality, and $x eq $y compares two strings.

There are a few exceptions though: x can be either string repetition or list repetition, depending on the type of the left operand, and & , | , ^ and ~ can be either string or numeric bit operations.

# Operator Precedence and Associativity

Operator precedence and associativity work in Perl more or less like they do in mathematics.

Operator precedence means some operators group more tightly than others. For example, in 2 + 4 * 5 , the multiplication has higher precedence, so 4 * 5 is grouped together as the right-hand operand of the addition, rather than 2 + 4 being grouped together as the left-hand operand of the multiplication. It is as if the expression were written 2 + (4 * 5) , not (2 + 4) * 5 . So the expression yields 2 + 20 == 22 , rather than 6 * 5 == 30 .

Operator associativity defines what happens if a sequence of the same operators is used one after another: usually that they will be grouped at the left or the right. For example, in 9 - 3 - 2 , subtraction is left associative, so 9 - 3 is grouped together as the left-hand operand of the second subtraction, rather than 3 - 2 being grouped together as the right-hand operand of the first subtraction. It is as if the expression were written (9 - 3) - 2 , not 9 - (3 - 2) . So the expression yields 6 - 2 == 4 , rather than 9 - 1 == 8 .

For simple operators that evaluate all their operands and then combine the values in some way, precedence and associativity (and parentheses) imply some ordering requirements on those combining operations. For example, in 2 + 4 * 5 , the grouping implied by precedence means that the multiplication of 4 and 5 must be performed before the addition of 2 and 20, simply because the result of that multiplication is required as one of the operands of the addition. But the order of operations is not fully determined by this: in 2 * 2 + 4 * 5 both multiplications must be performed before the addition, but the grouping does not say anything about the order in which the two multiplications are performed. In fact Perl has a general rule that the operands of an operator are evaluated in left-to-right order. A few operators such as &&= have special evaluation rules that can result in an operand not being evaluated at all; in general, the top-level operator in an expression has control of operand evaluation.

Some comparison operators, as their associativity, chain with some operators of the same precedence (but never with operators of different precedence). This chaining means that each comparison is performed on the two arguments surrounding it, with each interior argument taking part in two comparisons, and the comparison results are implicitly ANDed. Thus "$x < $y <= $z" behaves exactly like "$x < $y && $y <= $z" , assuming that "$y" is as simple a scalar as it looks. The ANDing short-circuits just like "&&" does, stopping the sequence of comparisons as soon as one yields false.

In a chained comparison, each argument expression is evaluated at most once, even if it takes part in two comparisons, but the result of the evaluation is fetched for each comparison. (It is not evaluated at all if the short-circuiting means that it's not required for any comparisons.) This matters if the computation of an interior argument is expensive or non-deterministic. For example,

is not entirely like

but instead closer to

in that the subroutine is only called once. However, it's not exactly like this latter code either, because the chained comparison doesn't actually involve any temporary variable (named or otherwise): there is no assignment. This doesn't make much difference where the expression is a call to an ordinary subroutine, but matters more with an lvalue subroutine, or if the argument expression yields some unusual kind of scalar by other means. For example, if the argument expression yields a tied scalar, then the expression is evaluated to produce that scalar at most once, but the value of that scalar may be fetched up to twice, once for each comparison in which it is actually used.

In this example, the expression is evaluated only once, and the tied scalar (the result of the expression) is fetched for each comparison that uses it.

In the next example, the expression is evaluated only once, and the tied scalar is fetched once as part of the operation within the expression. The result of that operation is fetched for each comparison, which normally doesn't matter unless that expression result is also magical due to operator overloading.

Some operators are instead non-associative, meaning that it is a syntax error to use a sequence of those operators of the same precedence. For example, "$x .. $y .. $z" is an error.

Perl operators have the following associativity and precedence, listed from highest precedence to lowest. Operators borrowed from C keep the same precedence relationship with each other, even where C's precedence is slightly screwy. (This makes learning Perl easier for C folks.) With very few exceptions, these all operate on scalar values only, not array values.

In the following sections, these operators are covered in detail, in the same order in which they appear in the table above.

Many operators can be overloaded for objects. See overload .

# Terms and List Operators (Leftward)

A TERM has the highest precedence in Perl. They include variables, quote and quote-like operators, any expression in parentheses, and any function whose arguments are parenthesized. Actually, there aren't really functions in this sense, just list operators and unary operators behaving as functions because you put parentheses around the arguments. These are all documented in perlfunc .

If any list operator ( print() , etc.) or any unary operator ( chdir() , etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call.

In the absence of parentheses, the precedence of list operators such as print , sort , or chmod is either very high or very low depending on whether you are looking at the left side or the right side of the operator. For example, in

the commas on the right of the sort are evaluated before the sort , but the commas on the left are evaluated after. In other words, list operators tend to gobble up all arguments that follow, and then act like a simple TERM with regard to the preceding expression. Be careful with parentheses:

Also note that

probably doesn't do what you expect at first glance. The parentheses enclose the argument list for print which is evaluated (printing the result of $foo & 255 ). Then one is added to the return value of print (usually 1). The result is something like this:

To do what you meant properly, you must write:

See "Named Unary Operators" for more discussion of this.

Also parsed as terms are the do {} and eval {} constructs, as well as subroutine and method calls, and the anonymous constructors [] and {} .

See also "Quote and Quote-like Operators" toward the end of this section, as well as "I/O Operators" .

# The Arrow Operator

" -> " is an infix dereference operator, just as it is in C and C++. If the right side is either a [...] , {...} , or a (...) subscript, then the left side must be either a hard or symbolic reference to an array, a hash, or a subroutine respectively. (Or technically speaking, a location capable of holding a hard reference, if it's an array or hash reference being used for assignment.) See perlreftut and perlref .

Otherwise, the right side is a method name or a simple scalar variable containing either the method name or a subroutine reference, and (if it is a method name) the left side must be either an object (a blessed reference) or a class name (that is, a package name). See perlobj .

The dereferencing cases (as opposed to method-calling cases) are somewhat extended by the postderef feature. For the details of that feature, consult "Postfix Dereference Syntax" in perlref .

# Auto-increment and Auto-decrement

"++" and "--" work as in C. That is, if placed before a variable, they increment or decrement the variable by one before returning the value, and if placed after, increment or decrement after returning the value.

Note that just as in C, Perl doesn't define when the variable is incremented or decremented. You just know it will be done sometime before or after the value is returned. This also means that modifying a variable twice in the same statement will lead to undefined behavior. Avoid statements like:

Perl will not guarantee what the result of the above statements is.

The auto-increment operator has a little extra builtin magic to it. If you increment a variable that is numeric, or that has ever been used in a numeric context, you get a normal increment. If, however, the variable has been used in only string contexts since it was set, and has a value that is not the empty string and matches the pattern /^[a-zA-Z]*[0-9]*\z/ , the increment is done as a string, preserving each character within its range, with carry:

undef is always treated as numeric, and in particular is changed to 0 before incrementing (so that a post-increment of an undef value will return 0 rather than undef ).

The auto-decrement operator is not magical.

# Exponentiation

Binary "**" is the exponentiation operator. It binds even more tightly than unary minus, so -2**4 is -(2**4) , not (-2)**4 . (This is implemented using C's pow(3) function, which actually works on doubles internally.)

Note that certain exponentiation expressions are ill-defined: these include 0**0 , 1**Inf , and Inf**0 . Do not expect any particular results from these special cases, the results are platform-dependent.

# Symbolic Unary Operators

Unary "!" performs logical negation, that is, "not". See also not for a lower precedence version of this.

Unary "-" performs arithmetic negation if the operand is numeric, including any string that looks like a number. If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier is returned. Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign is returned. One effect of these rules is that -bareword is equivalent to the string "-bareword" . If, however, the string begins with a non-alphabetic character (excluding "+" or "-" ), Perl will attempt to convert the string to a numeric, and the arithmetic negation is performed. If the string cannot be cleanly converted to a numeric, Perl will give the warning Argument "the string" isn't numeric in negation (-) at ... .

Unary "~" performs bitwise negation, that is, 1's complement. For example, 0666 & ~027 is 0640. (See also "Integer Arithmetic" and "Bitwise String Operators" .) Note that the width of the result is platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 bits wide on a 64-bit platform, so if you are expecting a certain bit width, remember to use the "&" operator to mask off the excess bits.

Starting in Perl 5.28, it is a fatal error to try to complement a string containing a character with an ordinal value above 255.

If the "bitwise" feature is enabled via use feature 'bitwise' or use v5.28 , then unary "~" always treats its argument as a number, and an alternate form of the operator, "~." , always treats its argument as a string. So ~0 and ~"0" will both give 2**32-1 on 32-bit platforms, whereas ~.0 and ~."0" will both yield "\xff" . Until Perl 5.28, this feature produced a warning in the "experimental::bitwise" category.

Unary "+" has no effect whatsoever, even on strings. It is useful syntactically for separating a function name from a parenthesized expression that would otherwise be interpreted as the complete list of function arguments. (See examples above under "Terms and List Operators (Leftward)" .)

Unary "\" creates references. If its operand is a single sigilled thing, it creates a reference to that object. If its operand is a parenthesised list, then it creates references to the things mentioned in the list. Otherwise it puts its operand in list context, and creates a list of references to the scalars in the list provided by the operand. See perlreftut and perlref . Do not confuse this behavior with the behavior of backslash within a string, although both forms do convey the notion of protecting the next thing from interpolation.

# Binding Operators

Binary "=~" binds a scalar expression to a pattern match. Certain operations search or modify the string $_ by default. This operator makes that kind of operation work on some other string. The right argument is a search pattern, substitution, or transliteration. The left argument is what is supposed to be searched, substituted, or transliterated instead of the default $_ . When used in scalar context, the return value generally indicates the success of the operation. The exceptions are substitution ( s/// ) and transliteration ( y/// ) with the /r (non-destructive) option, which cause the r eturn value to be the result of the substitution. Behavior in list context depends on the particular operator. See "Regexp Quote-Like Operators" for details and perlretut for examples using these operators.

If the right argument is an expression rather than a search pattern, substitution, or transliteration, it is interpreted as a search pattern at run time. Note that this means that its contents will be interpolated twice, so

is not ok, as the regex engine will end up trying to compile the pattern \ , which it will consider a syntax error.

Binary "!~" is just like "=~" except the return value is negated in the logical sense.

Binary "!~" with a non-destructive substitution ( s///r ) or transliteration ( y///r ) is a syntax error.

# Multiplicative Operators

Binary "*" multiplies two numbers.

Binary "/" divides two numbers.

Binary "%" is the modulo operator, which computes the division remainder of its first argument with respect to its second argument. Given integer operands $m and $n : If $n is positive, then $m % $n is $m minus the largest multiple of $n less than or equal to $m . If $n is negative, then $m % $n is $m minus the smallest multiple of $n that is not less than $m (that is, the result will be less than or equal to zero). If the operands $m and $n are floating point values and the absolute value of $n (that is abs($n) ) is less than (UV_MAX + 1) , only the integer portion of $m and $n will be used in the operation (Note: here UV_MAX means the maximum of the unsigned integer type). If the absolute value of the right operand ( abs($n) ) is greater than or equal to (UV_MAX + 1) , "%" computes the floating-point remainder $r in the equation ($r = $m - $i*$n) where $i is a certain integer that makes $r have the same sign as the right operand $n ( not as the left operand $m like C function fmod() ) and the absolute value less than that of $n . Note that when use integer is in scope, "%" gives you direct access to the modulo operator as implemented by your C compiler. This operator is not as well defined for negative operands, but it will execute faster.

Binary x is the repetition operator. In scalar context, or if the left operand is neither enclosed in parentheses nor a qw// list, it performs a string repetition. In that case it supplies scalar context to the left operand, and returns a string consisting of the left operand string repeated the number of times specified by the right operand. If the x is in list context, and the left operand is either enclosed in parentheses or a qw// list, it performs a list repetition. In that case it supplies list context to the left operand, and returns a list consisting of the left operand list repeated the number of times specified by the right operand. If the right operand is zero or negative (raising a warning on negative), it returns an empty string or an empty list, depending on the context.

# Additive Operators

Binary "+" returns the sum of two numbers.

Binary "-" returns the difference of two numbers.

Binary "." concatenates two strings.

# Shift Operators

Binary "<<" returns the value of its left argument shifted left by the number of bits specified by the right argument. Arguments should be integers. (See also "Integer Arithmetic" .)

Binary ">>" returns the value of its left argument shifted right by the number of bits specified by the right argument. Arguments should be integers. (See also "Integer Arithmetic" .)

If use integer (see "Integer Arithmetic" ) is in force then signed C integers are used ( arithmetic shift ), otherwise unsigned C integers are used ( logical shift ), even for negative shiftees. In arithmetic right shift the sign bit is replicated on the left, in logical shift zero bits come in from the left.

Either way, the implementation isn't going to generate results larger than the size of the integer type Perl was built with (32 bits or 64 bits).

Shifting by negative number of bits means the reverse shift: left shift becomes right shift, right shift becomes left shift. This is unlike in C, where negative shift is undefined.

Shifting by more bits than the size of the integers means most of the time zero (all bits fall off), except that under use integer right overshifting a negative shiftee results in -1. This is unlike in C, where shifting by too many bits is undefined. A common C behavior is "shift by modulo wordbits", so that for example

but that is completely accidental.

If you get tired of being subject to your platform's native integers, the use bigint pragma neatly sidesteps the issue altogether:

# Named Unary Operators

The various named unary operators are treated as functions with one argument, with optional parentheses.

If any list operator ( print() , etc.) or any unary operator ( chdir() , etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call. For example, because named unary operators are higher precedence than || :

but, because "*" is higher precedence than named operators:

Regarding precedence, the filetest operators, like -f , -M , etc. are treated like named unary operators, but they don't follow this functional parenthesis rule. That means, for example, that -f($file).".bak" is equivalent to -f "$file.bak" .

See also "Terms and List Operators (Leftward)" .

# Relational Operators

Perl operators that return true or false generally return values that can be safely used as numbers. For example, the relational operators in this section and the equality operators in the next one return 1 for true and a special version of the defined empty string, "" , which counts as a zero but is exempt from warnings about improper numeric conversions, just as "0 but true" is.

Binary "<" returns true if the left argument is numerically less than the right argument.

Binary ">" returns true if the left argument is numerically greater than the right argument.

Binary "<=" returns true if the left argument is numerically less than or equal to the right argument.

Binary ">=" returns true if the left argument is numerically greater than or equal to the right argument.

Binary "lt" returns true if the left argument is stringwise less than the right argument.

Binary "gt" returns true if the left argument is stringwise greater than the right argument.

Binary "le" returns true if the left argument is stringwise less than or equal to the right argument.

Binary "ge" returns true if the left argument is stringwise greater than or equal to the right argument.

A sequence of relational operators, such as "$x < $y <= $z" , performs chained comparisons, in the manner described above in the section "Operator Precedence and Associativity" . Beware that they do not chain with equality operators, which have lower precedence.

# Equality Operators

Binary "==" returns true if the left argument is numerically equal to the right argument.

Binary "!=" returns true if the left argument is numerically not equal to the right argument.

Binary "eq" returns true if the left argument is stringwise equal to the right argument.

Binary "ne" returns true if the left argument is stringwise not equal to the right argument.

A sequence of the above equality operators, such as "$x == $y == $z" , performs chained comparisons, in the manner described above in the section "Operator Precedence and Associativity" . Beware that they do not chain with relational operators, which have higher precedence.

Binary "<=>" returns -1, 0, or 1 depending on whether the left argument is numerically less than, equal to, or greater than the right argument. If your platform supports NaN 's (not-a-numbers) as numeric values, using them with "<=>" returns undef. NaN is not "<" , "==" , ">" , "<=" or ">=" anything (even NaN ), so those 5 return false. NaN != NaN returns true, as does NaN != anything else . If your platform doesn't support NaN 's then NaN is just a string with numeric value 0.

(Note that the bigint , bigrat , and bignum pragmas all support "NaN" .)

Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise less than, equal to, or greater than the right argument.

Here we can see the difference between <=> and cmp,

(likewise between gt and >, lt and <, etc.)

Binary "~~" does a smartmatch between its arguments. Smart matching is described in the next section.

The two-sided ordering operators "<=>" and "cmp" , and the smartmatch operator "~~" , are non-associative with respect to each other and with respect to the equality operators of the same precedence.

"lt" , "le" , "ge" , "gt" and "cmp" use the collation (sort) order specified by the current LC_COLLATE locale if a use locale form that includes collation is in effect. See perllocale . Do not mix these with Unicode, only use them with legacy 8-bit locale encodings. The standard Unicode::Collate and Unicode::Collate::Locale modules offer much more powerful solutions to collation issues.

For case-insensitive comparisons, look at the "fc" in perlfunc case-folding function, available in Perl v5.16 or later:

# Class Instance Operator

Binary isa evaluates to true when the left argument is an object instance of the class (or a subclass derived from that class) given by the right argument. If the left argument is not defined, not a blessed object instance, nor does not derive from the class given by the right argument, the operator evaluates as false. The right argument may give the class either as a bareword or a scalar expression that yields a string class name:

This feature is available from Perl 5.31.6 onwards when enabled by use feature 'isa' . This feature is enabled automatically by a use v5.36 (or higher) declaration in the current scope.

# Smartmatch Operator

First available in Perl 5.10.1 (the 5.10.0 version behaved differently), binary ~~ does a "smartmatch" between its arguments. This is mostly used implicitly in the when construct described in perlsyn , although not all when clauses call the smartmatch operator. Unique among all of Perl's operators, the smartmatch operator can recurse. The smartmatch operator is experimental and its behavior is subject to change.

It is also unique in that all other Perl operators impose a context (usually string or numeric context) on their operands, autoconverting those operands to those imposed contexts. In contrast, smartmatch infers contexts from the actual types of its operands and uses that type information to select a suitable comparison mechanism.

The ~~ operator compares its operands "polymorphically", determining how to compare them according to their actual types (numeric, string, array, hash, etc.). Like the equality operators with which it shares the same precedence, ~~ returns 1 for true and "" for false. It is often best read aloud as "in", "inside of", or "is contained in", because the left operand is often looked for inside the right operand. That makes the order of the operands to the smartmatch operand often opposite that of the regular match operator. In other words, the "smaller" thing is usually placed in the left operand and the larger one in the right.

The behavior of a smartmatch depends on what type of things its arguments are, as determined by the following table. The first row of the table whose types apply determines the smartmatch behavior. Because what actually happens is mostly determined by the type of the second operand, the table is sorted on the right operand instead of on the left.

The smartmatch implicitly dereferences any non-blessed hash or array reference, so the HASH and ARRAY entries apply in those cases. For blessed references, the Object entries apply. Smartmatches involving hashes only consider hash keys, never hash values.

The "like" code entry is not always an exact rendition. For example, the smartmatch operator short-circuits whenever possible, but grep does not. Also, grep in scalar context returns the number of matches, but ~~ returns only true or false.

Unlike most operators, the smartmatch operator knows to treat undef specially:

Each operand is considered in a modified scalar context, the modification being that array and hash variables are passed by reference to the operator, which implicitly dereferences them. Both elements of each pair are the same:

Two arrays smartmatch if each element in the first array smartmatches (that is, is "in") the corresponding element in the second array, recursively.

Because the smartmatch operator recurses on nested arrays, this will still report that "red" is in the array.

If two arrays smartmatch each other, then they are deep copies of each others' values, as this example reports:

If you were to set $b[3] = 4 , then instead of reporting that "a and b are deep copies of each other", it now reports that "b smartmatches in a" . That's because the corresponding position in @a contains an array that (eventually) has a 4 in it.

Smartmatching one hash against another reports whether both contain the same keys, no more and no less. This could be used to see whether two records have the same field names, without caring what values those fields might have. For example:

However, this only does what you mean if $init_fields is indeed a hash reference. The condition $init_fields ~~ $REQUIRED_FIELDS also allows the strings "name" , "rank" , "serial_num" as well as any array reference that contains "name" or "rank" or "serial_num" anywhere to pass through.

The smartmatch operator is most often used as the implicit operator of a when clause. See the section on "Switch Statements" in perlsyn .

# Smartmatching of Objects

To avoid relying on an object's underlying representation, if the smartmatch's right operand is an object that doesn't overload ~~ , it raises the exception " Smartmatching a non-overloaded object breaks encapsulation ". That's because one has no business digging around to see whether something is "in" an object. These are all illegal on objects without a ~~ overload:

However, you can change the way an object is smartmatched by overloading the ~~ operator. This is allowed to extend the usual smartmatch semantics. For objects that do have an ~~ overload, see overload .

Using an object as the left operand is allowed, although not very useful. Smartmatching rules take precedence over overloading, so even if the object in the left operand has smartmatch overloading, this will be ignored. A left operand that is a non-overloaded object falls back on a string or numeric comparison of whatever the ref operator returns. That means that

does not invoke the overload method with X as an argument. Instead the above table is consulted as normal, and based on the type of X , overloading may or may not be invoked. For simple strings or numbers, "in" becomes equivalent to this:

For example, this reports that the handle smells IOish (but please don't really do this!):

That's because it treats $fh as a string like "IO::Handle=GLOB(0x8039e0)" , then pattern matches against that.

# Bitwise And

Binary "&" returns its operands ANDed together bit by bit. Although no warning is currently raised, the result is not well defined when this operation is performed on operands that aren't either numbers (see "Integer Arithmetic" ) nor bitstrings (see "Bitwise String Operators" ).

Note that "&" has lower priority than relational operators, so for example the parentheses are essential in a test like

If the "bitwise" feature is enabled via use feature 'bitwise' or use v5.28 , then this operator always treats its operands as numbers. Before Perl 5.28 this feature produced a warning in the "experimental::bitwise" category.

# Bitwise Or and Exclusive Or

Binary "|" returns its operands ORed together bit by bit.

Binary "^" returns its operands XORed together bit by bit.

Although no warning is currently raised, the results are not well defined when these operations are performed on operands that aren't either numbers (see "Integer Arithmetic" ) nor bitstrings (see "Bitwise String Operators" ).

Note that "|" and "^" have lower priority than relational operators, so for example the parentheses are essential in a test like

If the "bitwise" feature is enabled via use feature 'bitwise' or use v5.28 , then this operator always treats its operands as numbers. Before Perl 5.28. this feature produced a warning in the "experimental::bitwise" category.

# C-style Logical And

Binary "&&" performs a short-circuit logical AND operation. That is, if the left operand is false, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated.

# C-style Logical Or

Binary "||" performs a short-circuit logical OR operation. That is, if the left operand is true, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated.

# C-style Logical Xor

Binary "^^" performs a logical XOR operation. Both operands are evaluated and the result is true only if exactly one of the operands is true. Scalar or list context propagates down to the right operand.

# Logical Defined-Or

Although it has no direct equivalent in C, Perl's // operator is related to its C-style "or". In fact, it's exactly the same as || , except that it tests the left hand side's definedness instead of its truth. Thus, EXPR1 // EXPR2 returns the value of EXPR1 if it's defined, otherwise, the value of EXPR2 is returned. ( EXPR1 is evaluated in scalar context, EXPR2 in the context of // itself). Usually, this is the same result as defined(EXPR1) ? EXPR1 : EXPR2 (except that the ternary-operator form can be used as a lvalue, while EXPR1 // EXPR2 cannot). This is very useful for providing default values for variables. If you actually want to test if at least one of $x and $y is defined, use defined($x // $y) .

The || , // and && operators return the last value evaluated (unlike C's || and && , which return 0 or 1). Thus, a reasonably portable way to find out the home directory might be:

In particular, this means that you shouldn't use this for selecting between two aggregates for assignment:

As alternatives to && and || when used for control flow, Perl provides the and and or operators (see below). The short-circuit behavior is identical. The precedence of "and" and "or" is much lower, however, so that you can safely use them after a list operator without the need for parentheses:

With the C-style operators that would have been written like this:

It would be even more readable to write that this way:

Using "or" for assignment is unlikely to do what you want; see below.

# Range Operators

Binary ".." is the range operator, which is really two different operators depending on the context. In list context, it returns a list of values counting (up by ones) from the left value to the right value. If the left value is greater than the right value then it returns the empty list. The range operator is useful for writing foreach (1..10) loops and for doing slice operations on arrays. In the current implementation, no temporary array is created when the range operator is used as the expression in foreach loops, but older versions of Perl might burn a lot of memory when you write something like this:

The range operator also works on strings, using the magical auto-increment, see below.

In scalar context, ".." returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of sed , awk , and various editors. Each ".." operator maintains its own boolean state, even across calls to a subroutine that contains it. It is false as long as its left operand is false. Once the left operand is true, the range operator stays true until the right operand is true, AFTER which the range operator becomes false again. It doesn't become false till the next time the range operator is evaluated. It can test the right operand and become false on the same evaluation it became true (as in awk ), but it still returns true once. If you don't want it to test the right operand until the next evaluation, as in sed , just use three dots ( "..." ) instead of two. In all other regards, "..." behaves just like ".." does.

The right operand is not evaluated while the operator is in the "false" state, and the left operand is not evaluated while the operator is in the "true" state. The precedence is a little lower than || and &&. The value returned is either the empty string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number in a range has the string "E0" appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1.

If either operand of scalar ".." is a constant expression, that operand is considered true if it is equal ( == ) to the current input line number (the $. variable).

To be pedantic, the comparison is actually int(EXPR) == int(EXPR) , but that is only an issue if you use a floating point expression; when implicitly using $. as described in the previous paragraph, the comparison is int(EXPR) == int($.) which is only an issue when $. is set to a floating point value and you are not reading from a file. Furthermore, "span" .. "spat" or 2.18 .. 3.14 will not do what you want in scalar context because each of the operands are evaluated using their integer representation.

As a scalar operator:

Here's a simple example to illustrate the difference between the two range operators:

This program will print only the line containing "Bar". If the range operator is changed to ... , it will also print the "Baz" line.

And now some examples as a list operator:

Because each operand is evaluated in integer form, 2.18 .. 3.14 will return two elements in list context.

The range operator in list context can make use of the magical auto-increment algorithm if both operands are strings, subject to the following rules:

With one exception (below), if both strings look like numbers to Perl, the magic increment will not be applied, and the strings will be treated as numbers (more specifically, integers) instead.

For example, "-2".."2" is the same as -2..2 , and "2.18".."3.14" produces 2, 3 .

The exception to the above rule is when the left-hand string begins with 0 and is longer than one character, in this case the magic increment will be applied, even though strings like "01" would normally look like a number to Perl.

For example, "01".."04" produces "01", "02", "03", "04" , and "00".."-1" produces "00" through "99" - this may seem surprising, but see the following rules for why it works this way. To get dates with leading zeros, you can say:

If you want to force strings to be interpreted as numbers, you could say

Note: In Perl versions 5.30 and below, any string on the left-hand side beginning with "0" , including the string "0" itself, would cause the magic string increment behavior. This means that on these Perl versions, "0".."-1" would produce "0" through "99" , which was inconsistent with 0..-1 , which produces the empty list. This also means that "0".."9" now produces a list of integers instead of a list of strings.

If the initial value specified isn't part of a magical increment sequence (that is, a non-empty string matching /^[a-zA-Z]*[0-9]*\z/ ), only the initial value will be returned.

For example, "ax".."az" produces "ax", "ay", "az" , but "*x".."az" produces only "*x" .

For other initial values that are strings that do follow the rules of the magical increment, the corresponding sequence will be returned.

For example, you can say

to get all normal letters of the English alphabet, or

to get a hexadecimal digit.

If the final value specified is not in the sequence that the magical increment would produce, the sequence goes until the next value would be longer than the final value specified. If the length of the final string is shorter than the first, the empty list is returned.

For example, "a".."--" is the same as "a".."zz" , "0".."xx" produces "0" through "99" , and "aaa".."--" returns the empty list.

As of Perl 5.26, the list-context range operator on strings works as expected in the scope of "use feature 'unicode_strings" . In previous versions, and outside the scope of that feature, it exhibits "The "Unicode Bug"" in perlunicode : its behavior depends on the internal encoding of the range endpoint.

Because the magical increment only works on non-empty strings matching /^[a-zA-Z]*[0-9]*\z/ , the following will only return an alpha:

To get the 25 traditional lowercase Greek letters, including both sigmas, you could use this instead:

However, because there are many other lowercase Greek characters than just those, to match lowercase Greek characters in a regular expression, you could use the pattern /(?:(?=\p{Greek})\p{Lower})+/ (or the experimental feature /(?[ \p{Greek} & \p{Lower} ])+/ ).

# Conditional Operator

Ternary "?:" is the conditional operator, just as in C. It works much like an if-then-else. If the argument before the ? is true, the argument before the : is returned, otherwise the argument after the : is returned. For example:

Scalar or list context propagates downward into the 2nd or 3rd argument, whichever is selected.

The operator may be assigned to if both the 2nd and 3rd arguments are legal lvalues (meaning that you can assign to them):

Because this operator produces an assignable result, using assignments without parentheses will get you in trouble. For example, this:

Really means this:

Rather than this:

That should probably be written more simply as:

# Assignment Operators

"=" is the ordinary assignment operator.

Assignment operators work as in C. That is,

is equivalent to

although without duplicating any side effects that dereferencing the lvalue might trigger, such as from tie() . Other assignment operators work similarly. The following are recognized:

Although these are grouped by family, they all have the precedence of assignment. These combined assignment operators can only operate on scalars, whereas the ordinary assignment operator can assign to arrays, hashes, lists and even references. (See "Context" and "List value constructors" in perldata , and "Assigning to References" in perlref .)

Unlike in C, the scalar assignment operator produces a valid lvalue. Modifying an assignment is equivalent to doing the assignment and then modifying the variable that was assigned to. This is useful for modifying a copy of something, like this:

Although as of 5.14, that can be also be accomplished this way:

Similarly, a list assignment in list context produces the list of lvalues assigned to, and a list assignment in scalar context returns the number of elements produced by the expression on the right hand side of the assignment.

The three dotted bitwise assignment operators ( &.= |.= ^.= ) are new in Perl 5.22. See "Bitwise String Operators" .

# Comma Operator

Binary "," is the comma operator. In scalar context it evaluates its left argument, throws that value away, then evaluates its right argument and returns that value. This is just like C's comma operator.

In list context, it's just the list argument separator, and inserts both its arguments into the list. These arguments are also evaluated from left to right.

The => operator (sometimes pronounced "fat comma") is a synonym for the comma except that it causes a word on its left to be interpreted as a string if it begins with a letter or underscore and is composed only of letters, digits and underscores. This includes operands that might otherwise be interpreted as operators, constants, single number v-strings or function calls. If in doubt about this behavior, the left operand can be quoted explicitly.

Otherwise, the => operator behaves exactly as the comma operator or list argument separator, according to context.

For example:

is equivalent to:

It is NOT :

The => operator is helpful in documenting the correspondence between keys and values in hashes, and other paired elements in lists.

The special quoting behavior ignores precedence, and hence may apply to part of the left operand:

That example prints something like "1314363215shiftbbb" , because the => implicitly quotes the shift immediately on its left, ignoring the fact that time.shift is the entire left operand.

# List Operators (Rightward)

On the right side of a list operator, the comma has very low precedence, such that it controls all comma-separated expressions found there. The only operators with lower precedence are the logical operators "and" , "or" , and "not" , which may be used to evaluate calls to list operators without the need for parentheses:

However, some people find that code harder to read than writing it with parentheses:

in which case you might as well just use the more customary "||" operator:

See also discussion of list operators in "Terms and List Operators (Leftward)" .

# Logical Not

Unary "not" returns the logical negation of the expression to its right. It's the equivalent of "!" except for the very low precedence.

# Logical And

Binary "and" returns the logical conjunction of the two surrounding expressions. It's equivalent to && except for the very low precedence. This means that it short-circuits: the right expression is evaluated only if the left expression is true.

# Logical or and Exclusive Or

Binary "or" returns the logical disjunction of the two surrounding expressions. It's equivalent to || except for the very low precedence. This makes it useful for control flow:

This means that it short-circuits: the right expression is evaluated only if the left expression is false. Due to its precedence, you must be careful to avoid using it as replacement for the || operator. It usually works out better for flow control than in assignments:

However, when it's a list-context assignment and you're trying to use || for control flow, you probably need "or" so that the assignment takes higher precedence.

Then again, you could always use parentheses.

Binary "xor" returns the exclusive-OR of the two surrounding expressions. It cannot short-circuit (of course).

There is no low precedence operator for defined-OR.

# C Operators Missing From Perl

Here is what C has that Perl doesn't:

Address-of operator. (But see the "\" operator for taking a reference.)

Dereference-address operator. (Perl's prefix dereferencing operators are typed: $ , @ , % , and & .)

Type-casting operator.

# Quote and Quote-like Operators

While we usually think of quotes as literal values, in Perl they function as operators, providing various kinds of interpolating and pattern matching capabilities. Perl provides customary quote characters for these behaviors, but also provides a way for you to choose your quote character for any of them. In the following table, a {} represents any pair of delimiters you choose.

Non-bracketing delimiters use the same character fore and aft, but the four sorts of ASCII brackets (round, angle, square, curly) all nest, which means that

is the same as

Note, however, that this does not always work for quoting Perl code:

is a syntax error. The Text::Balanced module (standard as of v5.8, and from CPAN before then) is able to do this properly.

If the extra_paired_delimiters feature is enabled, then Perl will additionally recognise a variety of Unicode characters as being paired. For a full list, see the "List of Extra Paired Delimiters" at the end of this document.

There can (and in some cases, must) be whitespace between the operator and the quoting characters, except when # is being used as the quoting character. q#foo# is parsed as the string foo , while q #foo# is the operator q followed by a comment. Its argument will be taken from the next line. This allows you to write:

The cases where whitespace must be used are when the quoting character is a word character (meaning it matches /\w/ ):

The following escape sequences are available in constructs that interpolate, and in transliterations whose delimiters aren't single quotes ( "'" ). In all the ones with braces, any number of blanks and/or tabs adjoining and within the braces are allowed (and ignored).

Note that any escape sequence using braces inside interpolated constructs may have optional blanks (tab or space characters) adjoining with and inside of the braces, as illustrated above by the second \x{ } example.

The result is the character specified by the hexadecimal number between the braces. See "[8]" below for details on which character.

Blanks (tab or space characters) may separate the number from either or both of the braces.

Otherwise, only hexadecimal digits are valid between the braces. If an invalid character is encountered, a warning will be issued and the invalid character and all subsequent characters (valid or invalid) within the braces will be discarded.

If there are no valid digits between the braces, the generated character is the NULL character ( \x{00} ). However, an explicit empty brace ( \x{} ) will not cause a warning (currently).

The result is the character specified by the hexadecimal number in the range 0x00 to 0xFF. See "[8]" below for details on which character.

Only hexadecimal digits are valid following \x . When \x is followed by fewer than two valid digits, any valid digits will be zero-padded. This means that \x7 will be interpreted as \x07 , and a lone "\x" will be interpreted as \x00 . Except at the end of a string, having fewer than two valid digits will result in a warning. Note that although the warning says the illegal character is ignored, it is only ignored as part of the escape and will still be used as the subsequent character in the string. For example:

The result is the Unicode character or character sequence given by name . See charnames .

\N{U+ hexadecimal number } means the Unicode character whose Unicode code point is hexadecimal number .

The character following \c is mapped to some other character as shown in the table:

In other words, it's the character whose code point has had 64 xor'd with its uppercase. \c? is DELETE on ASCII platforms because ord("?") ^ 64 is 127, and \c@ is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0.

Also, \c\ X yields chr(28) . " X " for any X , but cannot come at the end of a string, because the backslash would be parsed as escaping the end quote.

On ASCII platforms, the resulting characters from the list above are the complete set of ASCII controls. This isn't the case on EBCDIC platforms; see "OPERATOR DIFFERENCES" in perlebcdic for a full discussion of the differences between these for ASCII versus EBCDIC platforms.

Use of any other character following the "c" besides those listed above is discouraged, and as of Perl v5.20, the only characters actually allowed are the printable ASCII ones, minus the left brace "{" . What happens for any of the allowed other characters is that the value is derived by xor'ing with the seventh bit, which is 64, and a warning raised if enabled. Using the non-allowed characters generates a fatal error.

To get platform independent controls, you can use \N{...} .

The result is the character specified by the octal number between the braces. See "[8]" below for details on which character.

Otherwise, if a character that isn't an octal digit is encountered, a warning is raised, and the value is based on the octal digits before it, discarding it and all following characters up to the closing brace. It is a fatal error if there are no octal digits at all.

The result is the character specified by the three-digit octal number in the range 000 to 777 (but best to not use above 077, see next paragraph). See "[8]" below for details on which character.

Some contexts allow 2 or even 1 digit, but any usage without exactly three digits, the first being a zero, may give unintended results. (For example, in a regular expression it may be confused with a backreference; see "Octal escapes" in perlrebackslash .) Starting in Perl 5.14, you may use \o{} instead, which avoids all these problems. Otherwise, it is best to use this construct only for ordinals \077 and below, remembering to pad to the left with zeros to make three digits. For larger ordinals, either use \o{} , or convert to something else, such as to hex and use \N{U+} (which is portable between platforms with different character sets) or \x{} instead.

Several constructs above specify a character by a number. That number gives the character's position in the character set encoding (indexed from 0). This is called synonymously its ordinal, code position, or code point. Perl works on platforms that have a native encoding currently of either ASCII/Latin1 or EBCDIC, each of which allow specification of 256 characters. In general, if the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets it as a Unicode code point and the result is the corresponding Unicode character. For example \x{50} and \o{120} both are the number 80 in decimal, which is less than 256, so the number is interpreted in the native character set encoding. In ASCII the character in the 80th position (indexed from 0) is the letter "P" , and in EBCDIC it is the ampersand symbol "&" . \x{100} and \o{400} are both 256 in decimal, so the number is interpreted as a Unicode code point no matter what the native encoding is. The name of the character in the 256th position (indexed by 0) in Unicode is LATIN CAPITAL LETTER A WITH MACRON .

An exception to the above rule is that \N{U+ hex number } is always interpreted as a Unicode code point, so that \N{U+0050} is "P" even on EBCDIC platforms.

NOTE : Unlike C and other languages, Perl has no \v escape sequence for the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may use \N{VT} , \ck , \N{U+0b} , or \x0b . ( \v does have meaning in regular expression patterns in Perl, see perlre .)

The following escape sequences are available in constructs that interpolate, but not in transliterations.

See "quotemeta" in perlfunc for the exact definition of characters that are quoted by \Q .

\L , \U , \F , and \Q can stack, in which case you need one \E for each. For example:

If a use locale form that includes LC_CTYPE is in effect (see perllocale ), the case map used by \l , \L , \u , and \U is taken from the current locale. If Unicode (for example, \N{} or code points of 0x100 or beyond) is being used, the case map used by \l , \L , \u , and \U is as defined by Unicode. That means that case-mapping a single character can sometimes produce a sequence of several characters. Under use locale , \F produces the same results as \L for all locales but a UTF-8 one, where it instead uses the Unicode definition.

All systems use the virtual "\n" to represent a line terminator, called a "newline". There is no such thing as an unvarying, physical newline character. It is only an illusion that the operating system, device drivers, C libraries, and Perl all conspire to preserve. Not all systems read "\r" as ASCII CR and "\n" as ASCII LF. For example, on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed, and on systems without a line terminator, printing "\n" might emit no actual data. In general, use "\n" when you mean a "newline" for your system, but use the literal ASCII when you need an exact character. For example, most networking protocols expect and prefer a CR+LF ( "\015\012" or "\cM\cJ" ) for line terminators, and although they often accept just "\012" , they seldom tolerate just "\015" . If you get in the habit of using "\n" for networking, you may be burned some day.

For constructs that do interpolate, variables beginning with " $ " or " @ " are interpolated. Subscripted variables such as $a[3] or $href->{key}[0] are also interpolated, as are array and hash slices. But method calls such as $obj->meth are not.

Interpolating an array or slice interpolates the elements in order, separated by the value of $" , so is equivalent to interpolating join $", @array . "Punctuation" arrays such as @* are usually interpolated only if the name is enclosed in braces @{*} , but the arrays @_ , @+ , and @- are interpolated even without braces.

For double-quoted strings, the quoting from \Q is applied after interpolation and escapes are processed.

For the pattern of regex operators ( qr// , m// and s/// ), the quoting from \Q is applied after interpolation is processed, but before escapes are processed. This allows the pattern to match literally (except for $ and @ ). For example, the following matches:

Because $ or @ trigger interpolation, you'll need to use something like /\Quser\E\@\Qhost/ to match them literally.

Patterns are subject to an additional level of interpretation as a regular expression. This is done as a second pass, after variables are interpolated, so that regular expressions may be incorporated into the pattern from the variables. If this is not what you want, use \Q to interpolate a variable literally.

Apart from the behavior described above, Perl does not expand multiple levels of interpolation. In particular, contrary to the expectations of shell programmers, back-quotes do NOT interpolate within double quotes, nor do single quotes impede evaluation of variables when used within double quotes.

# Regexp Quote-Like Operators

Here are the quote-like operators that apply to pattern matching and related activities.

This operator quotes (and possibly compiles) its STRING as a regular expression. STRING is interpolated the same way as PATTERN in m/ PATTERN / . If "'" is used as the delimiter, no variable interpolation is done. Returns a Perl value which may be used instead of the corresponding / STRING /msixpodualn expression. The returned value is a normalized version of the original pattern. It magically differs from a string containing the same characters: ref(qr/x/) returns "Regexp"; however, dereferencing it is not well defined (you currently get the normalized version of the original pattern, but this may change).

For example,

The result may be used as a subpattern in a match:

Since Perl may compile the pattern at the moment of execution of the qr() operator, using qr() may have speed advantages in some situations, notably if the result of qr() is used standalone:

Precompilation of the pattern into an internal representation at the moment of qr() avoids the need to recompile the pattern every time a match /$pat/ is attempted. (Perl has many other internal optimizations, but none would be triggered in the above example if we did not use qr() operator.)

Options (specified by the following modifiers) are:

If a precompiled pattern is embedded in a larger pattern then the effect of "msixpluadn" will be propagated appropriately. The effect that the /o modifier has is not propagated, being restricted to those patterns explicitly using it.

The /a , /d , /l , and /u modifiers (added in Perl 5.14) control the character set rules, but /a is the only one you are likely to want to specify explicitly; the other three are selected automatically by various pragmas.

See perlre for additional information on valid syntax for STRING , and for a detailed look at the semantics of regular expressions. In particular, all modifiers except the largely obsolete /o are further explained in "Modifiers" in perlre . /o is described in the next section.

Searches a string for a pattern match, and in scalar context returns true if it succeeds, false if it fails. If no string is specified via the =~ or !~ operator, the $_ string is searched. (The string specified with =~ need not be an lvalue--it may be the result of an expression evaluation, but remember the =~ binds rather tightly.) See also perlre .

Options are as described in qr// above; in addition, the following match process modifiers are available:

If "/" is the delimiter then the initial m is optional. With the m you can use any pair of non-whitespace (ASCII) characters as delimiters. This is particularly useful for matching path names that contain "/" , to avoid LTS (leaning toothpick syndrome). If "?" is the delimiter, then a match-only-once rule applies, described in m? PATTERN ? below. If "'" (single quote) is the delimiter, no variable interpolation is performed on the PATTERN . When using a delimiter character valid in an identifier, whitespace is required after the m .

PATTERN may contain variables, which will be interpolated every time the pattern search is evaluated, except for when the delimiter is a single quote. (Note that $( , $) , and $| are not interpolated because they look like end-of-string tests.) Perl will not recompile the pattern unless an interpolated variable that it contains changes. You can force Perl to skip the test and never recompile by adding a /o (which stands for "once") after the trailing delimiter. Once upon a time, Perl would recompile regular expressions unnecessarily, and this modifier was useful to tell it not to do so, in the interests of speed. But now, the only reasons to use /o are one of:

The variables are thousands of characters long and you know that they don't change, and you need to wring out the last little bit of speed by having Perl skip testing for that. (There is a maintenance penalty for doing this, as mentioning /o constitutes a promise that you won't change the variables in the pattern. If you do change them, Perl won't even notice.)

you want the pattern to use the initial values of the variables regardless of whether they change or not. (But there are saner ways of accomplishing this than using /o .)

If the pattern contains embedded code, such as

then perl will recompile each time, even though the pattern string hasn't changed, to ensure that the current value of $x is seen each time. Use /o if you want to avoid this.

The bottom line is that using /o is almost never a good idea.

If the PATTERN evaluates to the empty string, the last successfully matched regular expression in the current dynamic scope is used instead (see also "Scoping Rules of Regex Variables" in perlvar ). In this case, only the g and c flags on the empty pattern are honored; the other flags are taken from the original pattern. If no match has previously succeeded, this will (silently) act instead as a genuine empty pattern (which will always match). Using a user supplied string as a pattern has the risk that if the string is empty that it triggers the "last successful match" behavior, which can be very confusing. In such cases you are recommended to replace m/$pattern/ with m/(?:$pattern)/ to avoid this behavior.

The last successful pattern may be accessed as a variable via ${^LAST_SUCCESSFUL_PATTERN} . Matching against it, or the empty pattern should have the same effect, with the exception that when there is no last successful pattern the empty pattern will silently match, whereas using the ${^LAST_SUCCESSFUL_PATTERN} variable will produce undefined warnings (if warnings are enabled). You can check defined(${^LAST_SUCCESSFUL_PATTERN}) to test if there is a "last successful match" in the current scope.

Note that it's possible to confuse Perl into thinking // (the empty regex) is really // (the defined-or operator). Perl is usually pretty good about this, but some pathological cases might trigger this, such as $x/// (is that ($x) / (//) or $x // / ?) and print $fh // ( print $fh(// or print($fh // ?). In all of these examples, Perl will assume you meant defined-or. If you meant the empty regex, just use parentheses or spaces to disambiguate, or even prefix the empty regex with an m (so // becomes m// ).

If the /g option is not used, m// in list context returns a list consisting of the subexpressions matched by the parentheses in the pattern, that is, ( $1 , $2 , $3 ...) (Note that here $1 etc. are also set). When there are no parentheses in the pattern, the return value is the list (1) for success. With or without parentheses, an empty list is returned upon failure.

This last example splits $foo into the first two words and the remainder of the line, and assigns those three fields to $F1 , $F2 , and $Etc . The conditional is true if any variables were assigned; that is, if the pattern matched.

The /g modifier specifies global pattern matching--that is, matching as many times as possible within the string. How it behaves depends on the context. In list context, it returns a list of the substrings matched by any capturing parentheses in the regular expression. If there are no parentheses, it returns a list of all the matched strings, as if there were parentheses around the whole pattern.

In scalar context, each execution of m//g finds the next match, returning true if it matches, and false if there is no further match. The position after the last match can be read or set using the pos() function; see "pos" in perlfunc . A failed match normally resets the search position to the beginning of the string, but you can avoid that by adding the /c modifier (for example, m//gc ). Modifying the target string also resets the search position.

You can intermix m//g matches with m/\G.../g , where \G is a zero-width assertion that matches the exact position where the previous m//g , if any, left off. Without the /g modifier, the \G assertion still anchors at pos() as it was at the start of the operation (see "pos" in perlfunc ), but the match is of course only attempted once. Using \G without /g on a target string that has not previously had a /g match applied to it is the same as using the \A assertion to match the beginning of the string. Note also that, currently, \G is only properly supported when anchored at the very beginning of the pattern.

Here's another way to check for sentences in a paragraph:

Here's how to use m//gc with \G :

The last example should print:

Notice that the final match matched q instead of p , which a match without the \G anchor would have done. Also note that the final match did not update pos . pos is only updated on a /g match. If the final match did indeed match p , it's a good bet that you're running an ancient (pre-5.6.0) version of Perl.

A useful idiom for lex -like scanners is /\G.../gc . You can combine several regexps like this to process a string part-by-part, doing different actions depending on which regexp matched. Each regexp tries to match where the previous one leaves off.

Here is the output (split into several lines):

This is just like the m/ PATTERN / search, except that it matches only once between calls to the reset() operator. This is a useful optimization when you want to see only the first occurrence of something in each file of a set of files, for instance. Only m?? patterns local to the current package are reset.

Another example switched the first "latin1" encoding it finds to "utf8" in a pod file:

The match-once behavior is controlled by the match delimiter being ? ; with any other delimiter this is the normal m// operator.

In the past, the leading m in m? PATTERN ? was optional, but omitting it would produce a deprecation warning. As of v5.22.0, omitting it produces a syntax error. If you encounter this construct in older code, you can just add m .

Searches a string for a pattern, and if found, replaces that pattern with the replacement text and returns the number of substitutions made. Otherwise it returns false (a value that is both an empty string ( "" ) and numeric zero ( 0 ) as described in "Relational Operators" ).

If the /r (non-destructive) option is used then it runs the substitution on a copy of the string and instead of returning the number of substitutions, it returns the copy whether or not a substitution occurred. The original string is never changed when /r is used. The copy will always be a plain string, even if the input is an object or a tied variable.

If no string is specified via the =~ or !~ operator, the $_ variable is searched and modified. Unless the /r option is used, the string specified must be a scalar variable, an array element, a hash element, or an assignment to one of those; that is, some sort of scalar lvalue.

If the delimiter chosen is a single quote, no variable interpolation is done on either the PATTERN or the REPLACEMENT . Otherwise, if the PATTERN contains a $ that looks like a variable rather than an end-of-string test, the variable will be interpolated into the pattern at run-time. If you want the pattern compiled only once the first time the variable is interpolated, use the /o option. If the pattern evaluates to the empty string, the last successfully executed regular expression is used instead. See perlre for further explanation on these.

Options are as with m// with the addition of the following replacement specific options:

Any non-whitespace delimiter may replace the slashes. Add space after the s when using a character allowed in identifiers. If single quotes are used, no interpretation is done on the replacement string (the /e modifier overrides this, however). Note that Perl treats backticks as normal delimiters; the replacement text is not evaluated as a command. If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own pair of quotes, which may or may not be bracketing quotes, for example, s(foo)(bar) or s<foo>/bar/ . A /e will cause the replacement portion to be treated as a full-fledged Perl expression and evaluated right then and there. It is, however, syntax checked at compile-time. A second e modifier will cause the replacement portion to be eval ed before being run as a Perl expression.

Note the use of $ instead of \ in the last example. Unlike sed , we use the \< digit > form only in the left hand side. Anywhere else it's $< digit >.

Occasionally, you can't use just a /g to get all the changes to occur that you might want. Here are two common cases:

While s/// accepts the /c flag, it has no effect beyond producing a warning if warnings are enabled.

# Quote-Like Operators

A single-quoted, literal string. A backslash represents a backslash unless followed by the delimiter or another backslash, in which case the delimiter or backslash is interpolated.

A double-quoted, interpolated string.

A string which is (possibly) interpolated and then executed as a system command, via /bin/sh or its equivalent if required. Shell wildcards, pipes, and redirections will be honored. Similarly to system , if the string contains no shell metacharacters then it will executed directly. The collected standard output of the command is returned; standard error is unaffected. In scalar context, it comes back as a single (potentially multi-line) string, or undef if the shell (or command) could not be started. In list context, returns a list of lines (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR ), or an empty list if the shell (or command) could not be started.

Because backticks do not affect standard error, use shell file descriptor syntax (assuming the shell supports this) if you care to address this. To capture a command's STDERR and STDOUT together:

To capture a command's STDOUT but discard its STDERR:

To capture a command's STDERR but discard its STDOUT (ordering is important here):

To exchange a command's STDOUT and STDERR in order to capture the STDERR but leave its STDOUT to come out the old STDERR:

To read both a command's STDOUT and its STDERR separately, it's easiest to redirect them separately to files, and then read from those files when the program is done:

The STDIN filehandle used by the command is inherited from Perl's STDIN. For example:

will print the sorted contents of the file named "stuff" .

Using single-quote as a delimiter protects the command from Perl's double-quote interpolation, passing it on to the shell instead:

How that string gets evaluated is entirely subject to the command interpreter on your system. On most platforms, you will have to protect shell metacharacters if you want them treated literally. This is in practice difficult to do, as it's unclear how to escape which characters. See perlsec for a clean and safe example of a manual fork() and exec() to emulate backticks safely.

On some platforms (notably DOS-like ones), the shell may not be capable of dealing with multiline commands, so putting newlines in the string may not get you what you want. You may be able to evaluate multiple commands in a single line by separating them with the command separator character, if your shell supports that (for example, ; on many Unix shells and & on the Windows NT cmd shell).

Perl will attempt to flush all files opened for output before starting the child process, but this may not be supported on some platforms (see perlport ). To be safe, you may need to set $| ( $AUTOFLUSH in English ) or call the autoflush() method of IO::Handle on any open handles.

Beware that some command shells may place restrictions on the length of the command line. You must ensure your strings don't exceed this limit after any necessary interpolations. See the platform-specific release notes for more details about your particular environment.

Using this operator can lead to programs that are difficult to port, because the shell commands called vary between systems, and may in fact not be present at all. As one example, the type command under the POSIX shell is very different from the type command under DOS. That doesn't mean you should go out of your way to avoid backticks when they're the right way to get something done. Perl was made to be a glue language, and one of the things it glues together is commands. Just understand what you're getting yourself into.

Like system , backticks put the child process exit code in $? . If you'd like to manually inspect failure, you can check all possible failure modes by inspecting $? like this:

Use the open pragma to control the I/O layers used when reading the output of the command, for example:

qx// can also be called like a function with "readpipe" in perlfunc .

See "I/O Operators" for more discussion.

Evaluates to a list of the words extracted out of STRING , using embedded whitespace as the word delimiters. It can be understood as being roughly equivalent to:

the differences being that it only splits on ASCII whitespace, generates a real list at compile time, and in scalar context it returns the last element in the list. So this expression:

is semantically equivalent to the list:

Some frequently seen examples:

A common mistake is to try to separate the words with commas or to put comments into a multi-line qw -string. For this reason, the use warnings pragma and the -w switch (that is, the $^W variable) produces warnings if the STRING contains the "," or the "#" character.

Transliterates all occurrences of the characters found (or not found if the /c modifier is specified) in the search list with the positionally corresponding character in the replacement list, possibly deleting some, depending on the modifiers specified. It returns the number of characters replaced or deleted. If no string is specified via the =~ or !~ operator, the $_ string is transliterated.

For sed devotees, y is provided as a synonym for tr .

If the /r (non-destructive) option is present, a new copy of the string is made and its characters transliterated, and this copy is returned no matter whether it was modified or not: the original string is always left unchanged. The new copy is always a plain string, even if the input string is an object or a tied variable.

Unless the /r option is used, the string specified with =~ must be a scalar variable, an array element, a hash element, or an assignment to one of those; in other words, an lvalue.

The characters delimitting SEARCHLIST and REPLACEMENTLIST can be any printable character, not just forward slashes. If they are single quotes ( tr' SEARCHLIST ' REPLACEMENTLIST ' ), the only interpolation is removal of \ from pairs of \\ ; so hyphens are interpreted literally rather than specifying a character range.

Otherwise, a character range may be specified with a hyphen, so tr/A-J/0-9/ does the same replacement as tr/ACEGIBDFHJ/0246813579/ .

If the SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST must have its own pair of quotes, which may or may not be bracketing quotes; for example, tr(aeiouy)(yuoiea) or tr[+\-*/]"ABCD" . This final example shows a way to visually clarify what is going on for people who are more familiar with regular expression patterns than with tr , and who may think forward slash delimiters imply that tr is more like a regular expression pattern than it actually is. (Another option might be to use tr[...][...] .)

tr isn't fully like bracketed character classes, just (significantly) more like them than it is to full patterns. For example, characters appearing more than once in either list behave differently here than in patterns, and tr lists do not allow backslashed character classes such as \d or \pL , nor variable interpolation, so "$" and "@" are always treated as literals.

The allowed elements are literals plus \' (meaning a single quote). If the delimiters aren't single quotes, also allowed are any of the escape sequences accepted in double-quoted strings. Escape sequence details are in the table near the beginning of this section .

A hyphen at the beginning or end, or preceded by a backslash is also always considered a literal. Precede a delimiter character with a backslash to allow it.

The tr operator is not equivalent to the tr(1) utility. tr[a-z][A-Z] will uppercase the 26 letters "a" through "z", but for case changing not confined to ASCII, use lc , uc , lcfirst , ucfirst (all documented in perlfunc ), or the substitution operator s/ PATTERN / REPLACEMENT / (with \U , \u , \L , and \l string-interpolation escapes in the REPLACEMENT portion).

Most ranges are unportable between character sets, but certain ones signal Perl to do special handling to make them portable. There are two classes of portable ranges. The first are any subsets of the ranges A-Z , a-z , and 0-9 , when expressed as literal characters.

capitalizes the letters "h" , "i" , "j" , and "k" and nothing else, no matter what the platform's character set is. In contrast, all of

do the same capitalizations as the previous example when run on ASCII platforms, but something completely different on EBCDIC ones.

The second class of portable ranges is invoked when one or both of the range's end points are expressed as \N{...}

removes from $string all the platform's characters which are equivalent to any of Unicode U+0020, U+0021, ... U+007D, U+007E. This is a portable range, and has the same effect on every platform it is run on. In this example, these are the ASCII printable characters. So after this is run, $string has only controls and characters which have no ASCII equivalents.

But, even for portable ranges, it is not generally obvious what is included without having to look things up in the manual. A sound principle is to use only ranges that both begin from, and end at, either ASCII alphabetics of equal case ( b-e , B-E ), or digits ( 1-4 ). Anything else is unclear (and unportable unless \N{...} is used). If in doubt, spell out the character sets in full.

If the /d modifier is specified, any characters specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note that this is slightly more flexible than the behavior of some tr programs, which delete anything they find in the SEARCHLIST , period.)

If the /s modifier is specified, sequences of characters, all in a row, that were transliterated to the same character are squashed down to a single instance of that character.

If the /d modifier is used, the REPLACEMENTLIST is always interpreted exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter than the SEARCHLIST , the final character, if any, is replicated until it is long enough. There won't be a final character if and only if the REPLACEMENTLIST is empty, in which case REPLACEMENTLIST is copied from SEARCHLIST . An empty REPLACEMENTLIST is useful for counting characters in a class, or for squashing character sequences in a class.

If the /c modifier is specified, the characters to be transliterated are the ones NOT in SEARCHLIST , that is, it is complemented. If /d and/or /s are also specified, they apply to the complemented SEARCHLIST . Recall, that if REPLACEMENTLIST is empty (except under /d ) a copy of SEARCHLIST is used instead. That copy is made after complementing under /c . SEARCHLIST is sorted by code point order after complementing, and any REPLACEMENTLIST is applied to that sorted result. This means that under /c , the order of the characters specified in SEARCHLIST is irrelevant. This can lead to different results on EBCDIC systems if REPLACEMENTLIST contains more than one character, hence it is generally non-portable to use /c with such a REPLACEMENTLIST .

Another way of describing the operation is this: If /c is specified, the SEARCHLIST is sorted by code point order, then complemented. If REPLACEMENTLIST is empty and /d is not specified, REPLACEMENTLIST is replaced by a copy of SEARCHLIST (as modified under /c ), and these potentially modified lists are used as the basis for what follows. Any character in the target string that isn't in SEARCHLIST is passed through unchanged. Every other character in the target string is replaced by the character in REPLACEMENTLIST that positionally corresponds to its mate in SEARCHLIST , except that under /s , the 2nd and following characters are squeezed out in a sequence of characters in a row that all translate to the same character. If SEARCHLIST is longer than REPLACEMENTLIST , characters in the target string that match a character in SEARCHLIST that doesn't have a correspondence in REPLACEMENTLIST are either deleted from the target string if /d is specified; or replaced by the final character in REPLACEMENTLIST if /d isn't specified.

Some examples:

If multiple transliterations are given for a character, only the first one is used:

will transliterate any A to X.

Because the transliteration table is built at compile time, neither the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote interpolation. That means that if you want to use variables, you must use an eval() :

A line-oriented form of quoting is based on the shell "here-document" syntax. Following a << you specify a string to terminate the quoted material, and all lines following the current line down to the terminating string are the value of the item.

Prefixing the terminating string with a ~ specifies that you want to use "Indented Here-docs" (see below).

The terminating string may be either an identifier (a word), or some quoted text. An unquoted identifier works like double quotes. There may not be a space between the << and the identifier, unless the identifier is explicitly quoted. The terminating string must appear by itself (unquoted and with no surrounding whitespace) on the terminating line.

If the terminating string is quoted, the type of quotes used determine the treatment of the text.

Double quotes indicate that the text will be interpolated using exactly the same rules as normal double quoted strings.

Single quotes indicate the text is to be treated literally with no interpolation of its content. This is similar to single quoted strings except that backslashes have no special meaning, with \\ being treated as two backslashes and not one as they would in every other quoting construct.

Just as in the shell, a backslashed bareword following the << means the same thing as a single-quoted string does:

This is the only form of quoting in perl where there is no need to worry about escaping content, something that code generators can and do make good use of.

The content of the here doc is treated just as it would be if the string were embedded in backticks. Thus the content is interpolated as though it were double quoted and then executed via the shell, with the results of the execution returned.

The here-doc modifier ~ allows you to indent your here-docs to make the code more readable:

This will print...

...with no leading whitespace.

The line containing the delimiter that marks the end of the here-doc determines the indentation template for the whole thing. Compilation croaks if any non-empty line inside the here-doc does not begin with the precise indentation of the terminating line. (An empty line consists of the single character "\n".) For example, suppose the terminating line begins with a tab character followed by 4 space characters. Every non-empty line in the here-doc must begin with a tab followed by 4 spaces. They are stripped from each line, and any leading white space remaining on a line serves as the indentation for that line. Currently, only the TAB and SPACE characters are treated as whitespace for this purpose. Tabs and spaces may be mixed, but are matched exactly; tabs remain tabs and are not expanded.

Additional beginning whitespace (beyond what preceded the delimiter) will be preserved:

Finally, the modifier may be used with all of the forms mentioned above:

And whitespace may be used between the ~ and quoted delimiters:

It is possible to stack multiple here-docs in a row:

Just don't forget that you have to put a semicolon on the end to finish the statement, as Perl doesn't know you're not going to try to do this:

If you want to remove the line terminator from your here-docs, use chomp() .

If you want your here-docs to be indented with the rest of the code, use the <<~FOO construct described under "Indented Here-docs" :

If you use a here-doc within a delimited construct, such as in s///eg , the quoted material must still come on the line following the <<FOO marker, which means it may be inside the delimited construct:

It works this way as of Perl 5.18. Historically, it was inconsistent, and you would have to write

outside of string evals.

Additionally, quoting rules for the end-of-string identifier are unrelated to Perl's quoting rules. q() , qq() , and the like are not supported in place of '' and "" , and the only interpolation is for backslashing the quoting character:

Finally, quoted strings cannot span multiple lines. The general rule is that the identifier must be a string literal. Stick with that, and you should be safe.

# Gory details of parsing quoted constructs

When presented with something that might have several different interpretations, Perl uses the DWIM (that's "Do What I Mean") principle to pick the most probable interpretation. This strategy is so successful that Perl programmers often do not suspect the ambivalence of what they write. But from time to time, Perl's notions differ substantially from what the author honestly meant.

This section hopes to clarify how Perl handles quoted constructs. Although the most common reason to learn this is to unravel labyrinthine regular expressions, because the initial steps of parsing are the same for all quoting operators, they are all discussed together.

The most important Perl parsing rule is the first one discussed below: when processing a quoted construct, Perl first finds the end of that construct, then interprets its contents. If you understand this rule, you may skip the rest of this section on the first reading. The other rules are likely to contradict the user's expectations much less frequently than this first one.

Some passes discussed below are performed concurrently, but because their results are the same, we consider them individually. For different quoting constructs, Perl performs different numbers of passes, from one to four, but these passes are always performed in the same order.

The first pass is finding the end of the quoted construct. This results in saving to a safe location a copy of the text (between the starting and ending delimiters), normalized as necessary to avoid needing to know what the original delimiters were.

If the construct is a here-doc, the ending delimiter is a line that has a terminating string as the content. Therefore <<EOF is terminated by EOF immediately followed by "\n" and starting from the first column of the terminating line. When searching for the terminating line of a here-doc, nothing is skipped. In other words, lines after the here-doc syntax are compared with the terminating string line by line.

For the constructs except here-docs, single characters are used as starting and ending delimiters. If the starting delimiter is an opening punctuation (that is ( , [ , { , or < ), the ending delimiter is the corresponding closing punctuation (that is ) , ] , } , or > ). If the starting delimiter is an unpaired character like / or a closing punctuation, the ending delimiter is the same as the starting delimiter. Therefore a / terminates a qq// construct, while a ] terminates both qq[] and qq]] constructs.

When searching for single-character delimiters, escaped delimiters and \\ are skipped. For example, while searching for terminating / , combinations of \\ and \/ are skipped. If the delimiters are bracketing, nested pairs are also skipped. For example, while searching for a closing ] paired with the opening [ , combinations of \\ , \] , and \[ are all skipped, and nested [ and ] are skipped as well. However, when backslashes are used as the delimiters (like qq\\ and tr\\\ ), nothing is skipped. During the search for the end, backslashes that escape delimiters or other backslashes are removed (exactly speaking, they are not copied to the safe location).

For constructs with three-part delimiters ( s/// , y/// , and tr/// ), the search is repeated once more. If the first delimiter is not an opening punctuation, the three delimiters must be the same, such as s!!! and tr))) , in which case the second delimiter terminates the left part and starts the right part at once. If the left part is delimited by bracketing punctuation (that is () , [] , {} , or <> ), the right part needs another pair of delimiters such as s(){} and tr[]// . In these cases, whitespace and comments are allowed between the two parts, although the comment must follow at least one whitespace character; otherwise a character expected as the start of the comment may be regarded as the starting delimiter of the right part.

During this search no attention is paid to the semantics of the construct. Thus:

do not form legal quoted expressions. The quoted part ends on the first " and / , and the rest happens to be a syntax error. Because the slash that terminated m// was followed by a SPACE , the example above is not m//x , but rather m// with no /x modifier. So the embedded # is interpreted as a literal # .

Also no attention is paid to \c\ (multichar control char syntax) during this search. Thus the second \ in qq/\c\/ is interpreted as a part of \/ , and the following / is not recognized as a delimiter. Instead, use \034 or \x1c at the end of quoted constructs.

The next step is interpolation in the text obtained, which is now delimiter-independent. There are multiple cases.

No interpolation is performed. Note that the combination \\ is left intact, since escaped delimiters are not available for here-docs.

No interpolation is performed at this stage. Any backslashed sequences including \\ are treated at the stage of "Parsing regular expressions" .

The only interpolation is removal of \ from pairs of \\ . Therefore "-" in tr''' and y''' is treated literally as a hyphen and no character range is available. \1 in the replacement of s''' does not work as $1 .

No variable interpolation occurs. String modifying combinations for case and quoting such as \Q , \U , and \E are not recognized. The other escape sequences such as \200 and \t and backslashed characters such as \\ and \- are converted to appropriate literals. The character "-" is treated specially and therefore \- is treated as a literal "-" .

\Q , \U , \u , \L , \l , \F (possibly paired with \E ) are converted to corresponding Perl constructs. Thus, "$foo\Qbaz$bar" is converted to $foo . (quotemeta("baz" . $bar)) internally. The other escape sequences such as \200 and \t and backslashed characters such as \\ and \- are replaced with appropriate expansions.

Let it be stressed that whatever falls between \Q and \E is interpolated in the usual way. Something like "\Q\\E" has no \E inside. Instead, it has \Q , \\ , and E , so the result is the same as for "\\\\E" . As a general rule, backslashes between \Q and \E may lead to counterintuitive results. So, "\Q\t\E" is converted to quotemeta("\t") , which is the same as "\\\t" (since TAB is not alphanumeric). Note also that:

may be closer to the conjectural intention of the writer of "\Q\t\E" .

Interpolated scalars and arrays are converted internally to the join and "." catenation operations. Thus, "$foo XXX '@arr'" becomes:

All operations above are performed simultaneously, left to right.

Because the result of "\Q STRING \E" has all metacharacters quoted, there is no way to insert a literal $ or @ inside a \Q\E pair. If protected by \ , $ will be quoted to become "\\\$" ; if not, it is interpreted as the start of an interpolated scalar.

Note also that the interpolation code needs to make a decision on where the interpolated scalar ends. For instance, whether "a $x -> {c}" really means:

Most of the time, the longest possible text that does not include spaces between components and which contains matching braces or brackets. because the outcome may be determined by voting based on heuristic estimators, the result is not strictly predictable. Fortunately, it's usually correct for ambiguous cases.

Processing of \Q , \U , \u , \L , \l , \F and interpolation happens as with qq// constructs.

It is at this step that \1 is begrudgingly converted to $1 in the replacement text of s/// , in order to correct the incorrigible sed hackers who haven't picked up the saner idiom yet. A warning is emitted if the use warnings pragma or the -w command-line flag (that is, the $^W variable) was set.

Processing of \Q , \U , \u , \L , \l , \F , \E , and interpolation happens (almost) as with qq// constructs.

Processing of \N{...} is also done here, and compiled into an intermediate form for the regex compiler. (This is because, as mentioned below, the regex compilation may be done at execution time, and \N{...} is a compile-time construct.)

However any other combinations of \ followed by a character are not substituted but only skipped, in order to parse them as regular expressions at the following step. As \c is skipped at this step, @ of \c@ in RE is possibly treated as an array symbol (for example @foo ), even though the same text in qq// gives interpolation of \c@ .

Code blocks such as (?{BLOCK}) are handled by temporarily passing control back to the perl parser, in a similar way that an interpolated array subscript expression such as "foo$array[1+f("[xyz")]bar" would be.

Moreover, inside (?{BLOCK}) , (?# comment ) , and a # -comment in a /x -regular expression, no processing is performed whatsoever. This is the first step at which the presence of the /x modifier is relevant.

Interpolation in patterns has several quirks: $| , $( , $) , @+ and @- are not interpolated, and constructs $var[SOMETHING] are voted (by several different estimators) to be either an array element or $var followed by an RE alternative. This is where the notation ${arr[$bar]} comes handy: /${arr[0-9]}/ is interpreted as array element -9 , not as a regular expression from the variable $arr followed by a digit, which would be the interpretation of /$arr[0-9]/ . Since voting among different estimators may occur, the result is not predictable.

The lack of processing of \\ creates specific restrictions on the post-processed text. If the delimiter is / , one cannot get the combination \/ into the result of this step. / will finish the regular expression, \/ will be stripped to / on the previous step, and \\/ will be left as is. Because / is equivalent to \/ inside a regular expression, this does not matter unless the delimiter happens to be character special to the RE engine, such as in s*foo*bar* , m[foo] , or m?foo? ; or an alphanumeric char, as in:

In the RE above, which is intentionally obfuscated for illustration, the delimiter is m , the modifier is mx , and after delimiter-removal the RE is the same as for m/ ^ a \s* b /mx . There's more than one reason you're encouraged to restrict your delimiters to non-alphanumeric, non-whitespace choices.

This step is the last one for all constructs except regular expressions, which are processed further.

Previous steps were performed during the compilation of Perl code, but this one happens at run time, although it may be optimized to be calculated at compile time if appropriate. After preprocessing described above, and possibly after evaluation if concatenation, joining, casing translation, or metaquoting are involved, the resulting string is passed to the RE engine for compilation.

Whatever happens in the RE engine might be better discussed in perlre , but for the sake of continuity, we shall do so here.

This is another step where the presence of the /x modifier is relevant. The RE engine scans the string from left to right and converts it into a finite automaton.

Backslashed characters are either replaced with corresponding literal strings (as with \{ ), or else they generate special nodes in the finite automaton (as with \b ). Characters special to the RE engine (such as | ) generate corresponding nodes or groups of nodes. (?#...) comments are ignored. All the rest is either converted to literal strings to match, or else is ignored (as is whitespace and # -style comments if /x is present).

Parsing of the bracketed character class construct, [...] , is rather different than the rule used for the rest of the pattern. The terminator of this construct is found using the same rules as for finding the terminator of a {} -delimited construct, the only exception being that ] immediately following [ is treated as though preceded by a backslash.

The terminator of runtime (?{...}) is found by temporarily switching control to the perl parser, which should stop at the point where the logically balancing terminating } is found.

It is possible to inspect both the string given to RE engine and the resulting finite automaton. See the arguments debug / debugcolor in the use re pragma, as well as Perl's -Dr command-line switch documented in "Command Switches" in perlrun .

This step is listed for completeness only. Since it does not change semantics, details of this step are not documented and are subject to change without notice. This step is performed over the finite automaton that was generated during the previous pass.

It is at this stage that split() silently optimizes /^/ to mean /^/m .

# I/O Operators

There are several I/O operators you should know about.

A string enclosed by backticks (grave accents) first undergoes double-quote interpolation. It is then interpreted as an external command, and the output of that command is the value of the backtick string, like in a shell. In scalar context, a single string consisting of all output is returned. In list context, a list of values is returned, one per line of output. (You can set $/ to use a different line terminator.) The command is executed each time the pseudo-literal is evaluated. The status value of the command is returned in $? (see perlvar for the interpretation of $? ). Unlike in csh , no translation is done on the return data--newlines remain newlines. Unlike in any of the shells, single quotes do not hide variable names in the command from interpretation. To pass a literal dollar-sign through to the shell you need to hide it with a backslash. The generalized form of backticks is qx// , or you can call the "readpipe" in perlfunc function. (Because backticks always undergo shell expansion as well, see perlsec for security concerns.)

In scalar context, evaluating a filehandle in angle brackets yields the next line from that file (the newline, if any, included), or undef at end-of-file or on error. When $/ is set to undef (sometimes known as file-slurp mode) and the file is empty, it returns '' the first time, followed by undef subsequently.

Ordinarily you must assign the returned value to a variable, but there is one situation where an automatic assignment happens. If and only if the input symbol is the only thing inside the conditional of a while statement (even if disguised as a for(;;) loop), the value is automatically assigned to the global variable $_ , destroying whatever was there previously. (This may seem like an odd thing to you, but you'll use the construct in almost every Perl script you write.) The $_ variable is not implicitly localized. You'll have to put a local $_; before the loop if you want that to happen. Furthermore, if the input symbol or an explicit assignment of the input symbol to a scalar is used as a while / for condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value.

Thus the following lines are equivalent:

This also behaves similarly, but assigns to a lexical variable instead of to $_ :

In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to see whether it is defined. The defined test avoids problems where the line has a string value that would be treated as false by Perl; for example a "" or a "0" with no trailing newline. If you really mean for such values to terminate the loop, they should be tested for explicitly:

In other boolean contexts, < FILEHANDLE > without an explicit defined test or comparison elicits a warning if the use warnings pragma or the -w command-line switch (the $^W variable) is in effect.

The filehandles STDIN, STDOUT, and STDERR are predefined. (The filehandles stdin , stdout , and stderr will also work except in packages, where they would be interpreted as local identifiers rather than global.) Additional filehandles may be created with the open() function, amongst others. See perlopentut and "open" in perlfunc for details on this.

If a < FILEHANDLE > is used in a context that is looking for a list, a list comprising all input lines is returned, one line per list element. It's easy to grow to a rather large data space this way, so use with care.

< FILEHANDLE > may also be spelled readline(* FILEHANDLE ) . See "readline" in perlfunc .

The null filehandle <> (sometimes called the diamond operator) is special: it can be used to emulate the behavior of sed and awk , and any other Unix filter program that takes a list of filenames, doing the same to each line of input from all of them. Input from <> comes either from standard input, or from each file listed on the command line. Here's how it works: the first time <> is evaluated, the @ARGV array is checked, and if it is empty, $ARGV[0] is set to "-" , which when opened gives you standard input. The @ARGV array is then processed as a list of filenames. The loop

is equivalent to the following Perl-like pseudo code:

except that it isn't so cumbersome to say, and will actually work. It really does shift the @ARGV array and put the current filename into the $ARGV variable. It also uses filehandle ARGV internally. <> is just a synonym for <ARGV> , which is magical. (The pseudo code above doesn't work because it treats <ARGV> as non-magical.)

Since the null filehandle uses the two argument form of "open" in perlfunc it interprets special characters, so if you have a script like this:

and call it with perl dangerous.pl 'rm -rfv *|' , it actually opens a pipe, executes the rm command and reads rm 's output from that pipe. If you want all items in @ARGV to be interpreted as file names, you can use the module ARGV::readonly from CPAN, or use the double diamond bracket:

Using double angle brackets inside of a while causes the open to use the three argument form (with the second argument being < ), so all arguments in ARGV are treated as literal filenames (including "-" ). (Note that for convenience, if you use <<>> and if @ARGV is empty, it will still read from the standard input.)

You can modify @ARGV before the first <> as long as the array ends up containing the list of filenames you really want. Line numbers ( $. ) continue as though the input were one big happy file. See the example in "eof" in perlfunc for how to reset line numbers on each file.

If you want to set @ARGV to your own list of files, go right ahead. This sets @ARGV to all plain text files if no @ARGV was given:

You can even set them to pipe commands. For example, this automatically filters compressed arguments through gzip :

If you want to pass switches into your script, you can use one of the Getopts modules or put a loop on the front like this:

The <> symbol will return undef for end-of-file only once. If you call it again after this, it will assume you are processing another @ARGV list, and if you haven't set @ARGV , will read input from STDIN.

If what the angle brackets contain is a simple scalar variable (for example, $foo ), then that variable contains the name of the filehandle to input from, or its typeglob, or a reference to the same. For example:

If what's within the angle brackets is neither a filehandle nor a simple scalar variable containing a filehandle name, typeglob, or typeglob reference, it is interpreted as a filename pattern to be globbed, and either a list of filenames or the next filename in the list is returned, depending on context. This distinction is determined on syntactic grounds alone. That means <$x> is always a readline() from an indirect handle, but <$hash{key}> is always a glob() . That's because $x is a simple scalar variable, but $hash{key} is not--it's a hash element. Even <$x > (note the extra space) is treated as glob("$x ") , not readline($x) .

One level of double-quote interpretation is done first, but you can't say <$foo> because that's an indirect filehandle as explained in the previous paragraph. (In older versions of Perl, programmers would insert curly brackets to force interpretation as a filename glob: <${foo}> . These days, it's considered cleaner to call the internal function directly as glob($foo) , which is probably the right way to have done it in the first place.) For example:

is roughly equivalent to:

except that the globbing is actually done internally using the standard File::Glob extension. Of course, the shortest way to do the above is:

A (file)glob evaluates its (embedded) argument only when it is starting a new list. All values must be read before it will start over. In list context, this isn't important because you automatically get them all anyway. However, in scalar context the operator returns the next value each time it's called, or undef when the list has run out. As with filehandle reads, an automatic defined is generated when the glob occurs in the test part of a while , because legal glob returns (for example, a file called 0 ) would otherwise terminate the loop. Again, undef is returned only once. So if you're expecting a single value from a glob, it is much better to say

because the latter will alternate between returning a filename and returning false.

If you're trying to do variable interpolation, it's definitely better to use the glob() function, because the older notation can cause people to become confused with the indirect filehandle notation.

If an angle-bracket-based globbing expression is used as the condition of a while or for loop, then it will be implicitly assigned to $_ . If either a globbing expression or an explicit assignment of a globbing expression to a scalar is used as a while / for condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value.

# Constant Folding

Like C, Perl does a certain amount of expression evaluation at compile time whenever it determines that all arguments to an operator are static and have no side effects. In particular, string concatenation happens at compile time between literals that don't do variable substitution. Backslash interpolation also happens at compile time. You can say

and this all reduces to one string internally. Likewise, if you say

the compiler precomputes the number which that expression represents so that the interpreter won't have to.

Perl doesn't officially have a no-op operator, but the bare constants 0 and 1 are special-cased not to produce a warning in void context, so you can for example safely do

# Bitwise String Operators

Bitstrings of any size may be manipulated by the bitwise operators ( ~ | & ^ ).

If the operands to a binary bitwise op are strings of different sizes, | and ^ ops act as though the shorter operand had additional zero bits on the right, while the & op acts as though the longer operand were truncated to the length of the shorter. The granularity for such extension or truncation is one or more bytes.

If you are intending to manipulate bitstrings, be certain that you're supplying bitstrings: If an operand is a number, that will imply a numeric bitwise operation. You may explicitly show which type of operation you intend by using "" or 0+ , as in the examples below.

This somewhat unpredictable behavior can be avoided with the "bitwise" feature, new in Perl 5.22. You can enable it via use feature 'bitwise' or use v5.28 . Before Perl 5.28, it used to emit a warning in the "experimental::bitwise" category. Under this feature, the four standard bitwise operators ( ~ | & ^ ) are always numeric. Adding a dot after each operator ( ~. |. &. ^. ) forces it to treat its operands as strings:

The assignment variants of these operators ( &= |= ^= &.= |.= ^.= ) behave likewise under the feature.

It is a fatal error if an operand contains a character whose ordinal value is above 0xFF, and hence not expressible except in UTF-8. The operation is performed on a non-UTF-8 copy for other operands encoded in UTF-8. See "Byte and Character Semantics" in perlunicode .

See "vec" in perlfunc for information on how to manipulate individual bits in a bit vector.

# Integer Arithmetic

By default, Perl assumes that it must do most of its arithmetic in floating point. But by saying

you may tell the compiler to use integer operations (see integer for a detailed explanation) from here to the end of the enclosing BLOCK. An inner BLOCK may countermand this by saying

which lasts until the end of that BLOCK. Note that this doesn't mean everything is an integer, merely that Perl will use integer operations for arithmetic, comparison, and bitwise operators. For example, even under use integer , if you take the sqrt(2) , you'll still get 1.4142135623731 or so.

Used on numbers, the bitwise operators ( & | ^ ~ << >> ) always produce integral results. (But see also "Bitwise String Operators" .) However, use integer still has meaning for them. By default, their results are interpreted as unsigned integers, but if use integer is in effect, their results are interpreted as signed integers. For example, ~0 usually evaluates to a large integral value. However, use integer; ~0 is -1 on two's-complement machines.

# Floating-point Arithmetic

While use integer provides integer-only arithmetic, there is no analogous mechanism to provide automatic rounding or truncation to a certain number of decimal places. For rounding to a certain number of digits, sprintf() or printf() is usually the easiest route. See perlfaq4 .

Floating-point numbers are only approximations to what a mathematician would call real numbers. There are infinitely more reals than floats, so some corners must be cut. For example:

Testing for exact floating-point equality or inequality is not a good idea. Here's a (relatively expensive) work-around to compare whether two floating-point numbers are equal to a particular number of decimal places. See Knuth, volume II, for a more robust treatment of this topic.

The POSIX module (part of the standard perl distribution) implements ceil() , floor() , and other mathematical and trigonometric functions. The Math::Complex module (part of the standard perl distribution) defines mathematical functions that work on both the reals and the imaginary numbers. Math::Complex is not as efficient as POSIX, but POSIX can't work with complex numbers.

Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself.

# Bigger Numbers

The standard Math::BigInt , Math::BigRat , and Math::BigFloat modules, along with the bignum , bigint , and bigrat pragmas, provide variable-precision arithmetic and overloaded operators, although they're currently pretty slow. At the cost of some space and considerable speed, they avoid the normal pitfalls associated with limited-precision representations.

Or with rationals:

Several modules let you calculate with unlimited or fixed precision (bound only by memory and CPU time). There are also some non-standard modules that provide faster implementations via external C libraries.

Here is a short, but incomplete summary:

Choose wisely.

# List of Extra Paired Delimiters

The complete list of accepted paired delimiters as of Unicode 14.0 is:

Perldoc Browser is maintained by Dan Book ( DBOOK ). Please contact him via the GitHub issue tracker or email regarding any issues with the site itself, search, or rendering of documentation.

The Perl documentation is maintained by the Perl 5 Porters in the development of Perl. Please contact them via the Perl issue tracker , the mailing list , or IRC to report any issues with the contents or format of the documentation.

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How do I use boolean variables in Perl?

I have tried:

None of these work. I get the error message

• 42 You might want to start with my book Learning Perl . It's easier than guessing what to do until you get it right (monkeys, typewriters, and Hamlet, and all that). :) –  brian d foy Commented Jun 24, 2009 at 17:30
• Over a decade later, Perl gets true and false "distinguished booleans" through the new builtin package –  brian d foy Commented Nov 2, 2023 at 16:46

10 Answers 10

Truth and Falsehood in man perlsyn explains:

The number 0, the strings '0' and "", the empty list "()", and "undef" are all false in a boolean context. All other values are true.

In Perl, the following evaluate to false in conditionals:

The rest are true. There are no barewords for true or false . (Note: Perl v5.38 introduced true and false through the new builtin pragma).

• 2 @BlueWaldo: you can also use cmp and <=> when comparing and assigning the results of the comparison to a scalar. $var = $var1 cmp $var2; 'cmp' and '<=>' (used for numeric comparisons) returns -1, 0, or 1 if left argument is less than, equal to, or greater than the right argument. Its not boolean but sometimes you may want to know if one argument ir equal or less than or greater than the other instead of just equal or not equal. –  user118435 Commented Jun 24, 2009 at 6:47
• 11 Problem 1: An empty list is not false, since it's impossible to check if an list is true or false. An empty list in scalar context returns undef . –  ikegami Commented Apr 13, 2011 at 20:40
• 5 Problem 2: ('') and '' are the same value. I think you wanted to imply a list with an element that consists of an empty string (even though parens don't create lists), but as I've already mentioned, it's impossible to check if a list is true of false. –  ikegami Commented Apr 13, 2011 at 20:42
• 6 Problem 3: Objects with an overloaded boolean operator can also be false. –  ikegami Commented Apr 13, 2011 at 20:43
• 16 @eternicode, Perl does have two specific value it uses when it needs to return true and false, so not only does it have booleans, it has true ( !0 aka PL_sv_yes ) and false ( !1 aka PL_sv_no ). Or are you saying Perl should croak whenever something other than these two values are tested for truthness? That would be completely awful. e.g. It would prevent $x ||= $default; –  ikegami Commented Apr 26, 2012 at 5:00

Since 5.36, you can use true and false from the builtin module/namespace. These are special true and false values that can be identified using is_bool . This is an experimental feature at this time.

But while these could be said to return the true and false, they are are but a true or false value respectively. In fact, every scalar is either true or false.

The most complete, concise definition of a false value I've come across is:

Anything that stringifies to the empty string or the string 0 is false. Everything else is true.

Therefore, the following values are false:

• The empty string.
• The string 0 .
• A signed integer with value zero.
• An unsigned integer with value zero.
• A floating point number with value positive zero.
• A floating point number with value negative zero.
• An undefined value.
• An object with an overloaded boolean operator that evaluates one of the above.
• A magical variable that evaluates to one of the above on fetch.

Any other scalar is true.

A note on "true zeroes"

While numbers that stringify to 0 are false, strings that numify to zero aren't necessarily. The only false strings are 0 and the empty string. Any other string, even if it numifies to zero, is true.

The following are strings that are true as a boolean and zero as a number:

• "0.0"
• "0E0"
• "00"
• "+0"
• "-0"
• " 0"
• "0\n"
• ".0"
• "0."
• "0 but true"
• "\t00"
• "\n0e1"
• "+0.e-9"
• Any string for which Scalar::Util::looks_like_number returns false. (e.g. "abc" )

• if I understood you right the word true in While numbers that stringify to 0 are true should be false or (to prevent confusion) evaluate to false . –  d.k Commented Aug 29, 2013 at 7:53
• 2 Your "concise" definition is inconsistent with your longer explanation. Consider: my $value = do { package XXX; use overload q[""] => sub { "XXX" }, q[bool] => sub { 0 }; bless [] }; . Now $value will stringify to "XXX" but boolifies to false. –  tobyink Commented Feb 25, 2014 at 15:17
• 1 @tobyink, The concise version isn't perfect, merely the best I've found. It's meant to be practical, not all-encompassing. Do note that value returned by your bool does stringify to 0 . Also, you are discouraged from creating inconsistent overloads, and the values you return could be considered such. (e.g. a && can be optimized into a || , so if these were inconsistent, you'd have a problem.) –  ikegami Commented Feb 25, 2014 at 15:30
• I'm not sure if 0x00 is covered here. –  Zaid Commented Feb 25, 2014 at 21:03
• 1 @Grinn, A list assignment in scalar context evaluates to the number of scalars to which its RHS evaluates. (See Scalar vs List Assignment Operator for details.) If the sub returns nothing, the assignment will evaluate to 0, which is false. If the sub returns at least one scalar, the assignment will evaluate to a positive number, which is true. –  ikegami Commented Feb 7, 2019 at 0:05

Perl doesn't have a native boolean type, but you can use comparison of integers or strings in order to get the same behavior. Alan's example is a nice way of doing that using comparison of integers. Here's an example

One thing that I've done in some of my programs is added the same behavior using a constant:

The lines marked in "use constant" define a constant named true that always evaluates to 1, and a constant named false that always evaluates by 0. Because of the way that constants are defined in Perl, the following lines of code fails as well:

The error message should say something like "Can't modify constant in scalar assignment."

I saw that in one of the comments you asked about comparing strings. You should know that because Perl combines strings and numeric types in scalar variables, you have different syntax for comparing strings and numbers:

The difference between these operators is a very common source of confusion in Perl.

• 6 use warnings; instead of #! perl -w –  Brad Gilbert Commented Jun 24, 2009 at 17:25
• 11 Using constants as a poor mans macros that way is dangerous. These code examples aren't equivalent: if ($exitstatus) { exit; } vs if ($exitstatus == true) { exit; } , which might not be obvious to a casual observer. (And yes, the last example is poor programming style, but that is beside the point). –  Zano Commented Nov 20, 2009 at 1:59

I recommend use boolean; . You have to install the boolean module from cpan though.

• 8 In Perl, as in life, there are many truths. The inexperienced like to write silly things like if ($my_true_value == true) . Pretending that there is One True Truth is, in my experience, a path to pain, and inefficient code. –  tjd Commented Dec 4, 2014 at 17:02
• 2 Perl is philosophical by nature –  ILMostro_7 Commented Dec 1, 2015 at 10:26

My favourites have always been

which is completely independent from the internal representation.

• 6 Brilliant. You single-handedly fixed the Perl programming language! –  Nostalg.io Commented Sep 28, 2017 at 18:15

I came across a tutorial which have a well explaination about What values are true and false in Perl . It state that:

Following scalar values are considered false:

• undef - the undefined value
• 0 the number 0, even if you write it as 000 or 0.0
• '' the empty string.
• '0' the string that contains a single 0 digit.

All other scalar values, including the following are true:

• 1 any non-0 number
• ' ' the string with a space in it
• '00' two or more 0 characters in a string
• "0\n" a 0 followed by a newline
• 'false' yes, even the string 'false' evaluates to true.

There is another good tutorial which explain about Perl true and false .

Beautiful explanation given by bobf for Boolean values : True or False? A Quick Reference Guide

Truth tests for different values

• 1 Poor answer, but a strong, reputable source in perlmonks.org. It would be nice to have some real content instead of a comment and a link. :-/ –  ILMostro_7 Commented Dec 1, 2015 at 10:30

use the following file prefix, this will add to your perl script eTRUE and eFALSE, it will actually be REAL(!) true and false (just like java)

There are, actually, few reasons why you should use that.

My reason is that working with JSON, I've got 0 and 1 as values to keys, but this hack will make sure correct values are kept along your script.

Perl v5.38 introduced experimental "distinguished booleans" through the new builtin pragma. This means that you now have the true or false that you wanted.

Booleans in Raku (the programming language formerly known as Perl_6):

https://docs.raku.org/type/Bool https://docs.raku.org/language/syntax#index-entry-Boolean_(literals)

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Home » Perl Operators

Perl Operators

Summary : in this tutorial, you’ll learn about Perl operators including numeric operators, string operators, and logical operators.

Numeric operators

Perl provides numeric operators to help you operate on numbers including arithmetic, Boolean and bitwise operations. Let’s examine the different kinds of operators in more detail.

Arithmetic operators

Perl arithmetic operators deal with basic math such as adding, subtracting, multiplying, diving, etc. To add (+ ) or subtract (-) numbers, you would do something as follows:

To multiply or divide numbers, you use divide (/) and multiply (*) operators as follows:

When you combine adding, subtracting, multiplying, and dividing operators together, Perl will perform the calculation in an order, which is known as operator precedence.

The multiply and divide operators have higher precedence than add and subtract operators, therefore, Perl performs multiplying and dividing before adding and subtracting. See the following example:

Perl performs 20/2 and 5*2 first, therefore you will get 10 + 10 – 10 = 10.

You can use brackets () to force Perl to perform calculations based on the precedence you want as shown in the following example:

To raise one number to the power of another number, you use the exponentiation operator (**) e.g., 2**3 = 2 * 2 * 2. The following example demonstrates the exponentiation operators:

To get the remainder of the division of one number by another, you use the modulo operator (%).

It is handy to use the modulo operator (%) to check if a number is odd or even by dividing it by 2 to get the remainder. If the remainder is zero, the number is even, otherwise, the number is odd. See the following example:

Bitwise Operators

Bitwise operators allow you to operate on numbers one bit at a time. Think of a number as a series of bits e.g., 125 can be represented in binary form as 1111101 . Perl provides all basic bitwise operators including and (&), or (|), exclusive or (^) , not (~) operators, shift right (>>), and shift left (<<) operators.

The bitwise operators perform from right to left. In other words, bitwise operators perform from the rightmost bit to the leftmost bit.

The following example demonstrates all bitwise operators:

If you are not familiar with bitwise operations, we are highly recommended you check out bitwise operations on Wikipedia .

Comparison operators for numbers

Perl provides all comparison operators for numbers as listed in the following table:

Equality	Operators
Equal	==
Not Equal	!=
Comparison	<=>
Less than	<
Greater than	>
Less than or equal	<=
Greater than or equal	>=

All the operators in the table above are obvious except the number comparison operator  <=> which is also known as spaceship operator.

The number comparison operator is often used in sorting numbers. See the code below:

The number operator returns:

• 1 if $a is greater than $b
• 0 if $a and $b are equal
• -1 if $a is lower than $b

Take a look at the following example:

String operators

String comparison operators.

Perl provides the corresponding comparison operators for strings. Let’s take a look a the table below:

Equality	Operators
Equal	eq
Not Equal	ne
Comparison	cmp
Less than	lt
Greater than	gt
Less than or equal	le
Greater than or equal	ge

String concatenation operators

Perl provides the concatenation ( . ) and repetition ( x ) operators that allow you to manipulate strings. Let’s take a look at the concatenation operator (.) first:

The concatenation operator (.) combines two strings together.

A string can be repeated with the repetition ( x ) operator:

The chomp() operator

The chomp() operator (or function ) removes the last character in a string and returns a number of characters that were removed. The chomp() operator is very useful when dealing with the user’s input because it helps you remove the new line character \n from the string that the user entered.

The <STDIN> is used to get input from users.

Logical operators

Logical operators are often used in control statements such as if , while ,  given, etc.,   to control the flow of the program. The following are logical operators in Perl:

• $a && $b performs the logic AND of two variables or expressions. The logical && operator checks if both variables or expressions are true.
• $a || $b performs the logic OR of two variables or expressions. The logical || operator checks whether a variable or expression is true.
• !$a performs the logic NOT of the variable or expression. The logic ! operator inverts the value of the following variable or expression. In the other words, it converts true to false or false to true .

You will learn how to use logical operators in conditional statements such as  if ,  while  and  given .

In this tutorial, you’ve learned some basic Perl operators. These operators are very important so make sure that you get familiar with them.

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Perl Operator: A Comprehensive Overview

Discover this blog to explore an in-depth analysis of Perl Operator. This blog will also cover different types, including arithmetic, string, logical, and bitwise operators, explaining their syntax and usage. This blog also emphasises their role in controlling program flow and manipulating data, highlighting best practices for efficient coding in Perl.

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Perl is a versatile scripting language that offers a rich set of operators and essential tools for programmers to manipulate data and control program flow. Perl Operators are categorised into various types, such as arithmetic for mathematical operations, string operators for text manipulation, logical operators for decision-making, and more. Each type is unique, allowing for addition, concatenation, comparison, and condition evaluation.   

Understanding these operators is crucial for effective Perl programming, as they are fundamental in constructing expressions and enabling complex functionalities. Do you want to learn more about these Operators? Read this blog to learn more about Perl Operators and explore these powerful Operators and how they are used. 

Table of Contents 

1) What is Perl Operator? 

2) Arithmetic Operators 

3) Equality Operators 

4) Assignment Operators 

5) Bitwise Operators 

6) Logical Operators 

7) Conclusion 

What is a Perl Operator? 

Understanding Perl Operators is essential for any programmer working with the language. They allow for executing complex operations and control structures, rendering Perl an effective tool for various applications, from web development to system administration.   

In Perl, an Operator is a key element for executing specific operations on operands, which can be values or variables. These Operators are integral to forming expressions in Perl, enabling data manipulation and dictating program logic flow. The richness and variety of operators in Perl make it a versatile language suitable for various programming tasks.   

Operators in Perl are categorised into several types, each with its unique role. Arithmetic Operators, for instance, are used for basic mathematical operations like addition, subtraction, multiplication, and division. 

These are fundamental for any calculations within the program. String operators, on the other hand, are designed specifically for string manipulation, allowing for operations like concatenation, repetition, and string comparison. 

  

Arithmetic Operators 

Arithmetic operators, including Perl, are fundamental in any programming language as they allow for basic mathematical operations within the code. These operators enable the manipulation of numerical values, forming the backbone of many computational tasks. In Perl, arithmetic operators are intuitive and follow conventional mathematical notation, making them accessible to both beginners and experienced programmers.   

a)  Addition (+): The most basic arithmetic operator is addition, represented by the plus sign (+). It is used to add two or more numbers. For example, ‘$sum = $a + $b’; will add the values of ‘$a’ and ‘$b’ and store the result in ‘$sum’. 

my $a = 5; 

my $b = 3; 

my $sum = $a + $b;  

# $sum is 8 

b) Subtraction (-): The subtraction operator, denoted by a minus sign (-), subtracts one number from another. For instance, ‘$difference = $a - $b’; will subtract ‘$b’ from ‘$a’ 

my $a = 10; 

my $b = 4; 

my $difference = $a - $b;  

# $difference is 6 

c) Multiplication (*): Represented by an asterisk (*), the multiplication operator is used to multiply two numbers. ‘$product = $a * $b’; multiplies ‘$a’ and ‘$b’. 

my $a = 7; 

my $b = 6; 

my $product = $a * $b; 

# $product is 42 

d) Division (/): The division operator, symbolised by a forward slash (/), divides one number by another. ‘$quotient = $a / $b’; will divide ‘$a’ by ‘$b’. 

my $a = 20; 

my $b = 5; 

my $quotient = $a / $b;  

# $quotient is 4 

e) Modulus (%): The modulus operator, represented by a percent sign (%), is used to find the remainder of a division operation. ‘$remainder = $a % $b’; will give the remainder when ‘$a’ is divided by ‘$b’. 

my $remainder = $a % $b;  

# $remainder is 1 

f) Exponentiation (): Perl also supports exponentiation (raising a number to the power of another number) using the double asterisk operator (). ‘$power = $a ** $b’; will raise ‘$a’ to the power of ‘$b’. 

my $a = 2; 

my $power = $a ** $b;  

# $power is 8 

g) Auto-increment (++) and Auto-decrement (--): Perl provides two very useful arithmetic operators: auto-increment (++) and auto-decrement (--). The auto-increment operator increases a number's value by one, while the auto-decrement operator decreases it by one. These operators can be used both as pre- and post-operators. 

my $value = 5; 

$value++; # $value is now 6 

$value--; # $value is back to 5 

h) Assignment with Arithmetic Operators: Perl combines assignment with arithmetic operators, enabling shorthand operations. For example, $a += $b; is equivalent to $a = $a + $b;. 

$a += 5; # $a is now 15 

$a *= 2; # $a is now 30 

Equality Operators 

In Perl, Equality Operators are essential for comparing values, a fundamental aspect of programming that allows for decision-making based on conditions. These operators test for equality or inequality, yielding Boolean values (true or false). Understanding equality operators is crucial for controlling program flow through conditional statements like ‘if’, ‘unless’, ‘while’, and others. 

Types of Equality Operators in Perl: 

Perl distinguishes between two types of equality comparisons: numeric and string. This distinction is crucial because Perl is a context-sensitive language, meaning it treats the same data differently based on the context. 

a) Numeric Equality Operators 

1) Equal (==): This operator checks if two numbers are equal. If they are, it returns true; otherwise, it returns false. 

if ($a == $b) { 

    print "a and b are equal"; 

2) Not equal (!=): It tests whether two numbers are not equal. If they are different, it returns true. 

if ($a != $b) { 

    print "a and b are not equal"; 

b) String Equality Operators 

1.    Equal (eq): This operator checks whether two strings are identical. It's essential to use eq instead of == when comparing strings. 

if ($string1 eq $string2) { 

    print "The strings are equal"; 

2.    Not equal (ne): It checks if two strings are different. 

if ($string1 ne $string2) { 

    print "The strings are not equal"; 

Assignment Operators 

Assignment operators in programming languages like Perl are fundamental components that simplify assigning values to variables. They are not just limited to the basic assignment but also include a range of compound assignment operators that combine arithmetic, string, and other operations with the assignment. Understanding these operators is crucial for writing concise and efficient code. 

a)  Basic Assignment Operator 

1) Equals (=): The most basic assignment operator is the equals sign (=). It assigns the value on its right to the variable on its left.  

my $a = 5;  # Assigns 5 to $a 

b) Compound Assignment Operators: 

Perl enhances the functionality of the basic assignment operator with compound assignment operators, which combine an operation with the assignment.   

1)  Addition Assignment (+=): Adds the right operand to the left operand and assigns the result to the left operand. 

 $a += 3;  # Equivalent to $a = $a + 3 

2) Subtraction Assignment (-=): Subtracts the right operand from the left operand and assigns the result to the left operand. 

 $a -= 2;  # Equivalent to $a = $a - 2 

3) Multiplication Assignment (*=): Multiplies the left operand by the right operand and assigns the result to the left operand. 

 $a *= 4;  # Equivalent to $a = $a * 4 

4) Division Assignment (/=): Divides the left operand by the right operand and assigns the result to the left operand. 

 $a /= 2;  # Equivalent to $a = $a / 2 

5) Modulus Assignment (%=): Applies modulus operation and assigns the result to the left operand. 

 $a %= 3;  # Equivalent to $a = $a % 3 

6) Exponentiation Assignment (**=): Raises the left operand to the power of the right operand and assigns the result back. 

 $a **= 2;  # Equivalent to $a = $a ** 2 

7) String Concatenation Assignment (.=): Appends the right string operand to the left string operand.  

my $str = "Hello"; 

$str .= " World";  # $str now is "Hello World" 

8) Bitwise AND Assignment (&=), Bitwise OR Assignment (|=), and Bitwise XOR Assignment (^=): These perform the corresponding bitwise operation and assign the result to the left operand. 

$a &= $b;  # Bitwise AND 

$a |= $b;  # Bitwise OR 

$a ^= $b;  # Bitwise XOR 

Bitwise Operators 

Bitwise Operators are a category of operators in programming languages, such as Perl, that perform operations at the bit level on numeric values. These operators treat their operands as sequences of 32 or 64 bits (depending on the platform) and operate on them bit by bit.  

Understanding bitwise operators is essential for tasks involving low-level data manipulation, such as working with binary data, flags, and masks.  

Types of Bitwise Operators: 

1) AND (&): The bitwise AND operator compares each bit of its first operand to the corresponding bit of its second operand. If both bits are 1, it sets the bit in the result to 1; otherwise, it is 0. 

 $result = $a & $b;  # Bitwise AND of $a and $b 

2) OR (|): This operator compares each bit of its first operand to the corresponding bit of its second operand. If either bit is 1, it sets the result bit to 1. 

 $result = $a | $b;  # Bitwise OR of $a and $b 

3) XOR (^): The bitwise XOR (exclusive OR) operator compares each bit of its first operand to the corresponding bit of its second operand. If the bits are different, it sets the result bit to 1. 

 $result = $a ^ $b;  # Bitwise XOR of $a and $b 

4) NOT (~): The bitwise NOT operator inverts all the bits of its operand. 

 $result = ~$a;  # Bitwise NOT of $a 

5) Left Shift ( This operator shifts all bits of its first operand to the left by the number of places specified in the second operand. New bits on the right are filled with zeros. 

 $result = $a << $shift;  # Left shift $a by $shift bits 

6) Right Shift (>>): It shifts all bits of its first operand to the right. The behaviour for the leftmost bits depends on the number type and whether it's signed or unsigned. 

 $result = $a >> $shift;  # Right shift $a by $shift bits 

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Logical Operators 

Logical operators in Perl are crucial for constructing logical expressions, which are fundamental to controlling program flow through conditional statements like ‘if’, ‘while’, and ‘unless’. These operators evaluate expressions and return Boolean values (true or false) based on the logic they implement. The primary logical operators in Perl are ‘and’, ‘or’, ‘not’, ‘&&’, ‘||’, and ‘!’.   

Types of Logical Operators: 

1) AND (&& and ‘and’): This operator returns true if both operands are true. The difference between ‘&’ and ‘and’ is their precedence, with && having a higher precedence. 

 if ($a && $b) { 

    # Executes if both $a and $b are true 

2) OR (|| and or): The OR operator returns true if either of its operands is true. Similar to AND, ‘||’ has a higher precedence than ‘or’.  

if ($a || $b) { 

    # Executes if either $a or $b is true 

3) NOT (! and not): NOT is a unary operator that inverts the truth value of its operand. ! has a higher precedence than not. 

 if (!$a) { 

    # Executes if $a is not true (i.e., false) 

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Conclusion 

Perl Operators are fundamental tools that greatly enhance the language's power and flexibility. They enable efficient data manipulation, logical decision-making, and control over program flow. Mastering these operators, from arithmetic to logical, is crucial for effective Perl scripting, allowing for concise and powerful code in various applications. 

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Frequently Asked Questions

The most commonly used Perl operators are:  

a) Arithmetic Operators (+, -, *, /, %) for basic mathematical operations. 

b) String Operators (. for concatenation, x for repetition) for string manipulation. 

c) Logical Operators (&&, ||, !) for evaluating Boolean expressions and controlling program flow.   

Perl uses different operators for numeric and string comparison. For numeric comparison, it uses ‘==’ (equal), ‘!=’ (not equal), ‘ ’ (greater than), etc. For string comparison, it uses ‘eq’ (equal), ‘ne’ (not equal), ‘lt’ (less than), ‘gt’ (greater than), etc. This distinction ensures accurate comparisons based on the data type. 

Yes, Perl operators can be combined to form complex expressions. For instance, arithmetic operators can be used with assignment operators (like +=, *=) for compound assignments. Logical operators can be used to combine multiple conditions in control structures. However, it's important to remember operator precedence and use parentheses for clarity when needed. 

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• Table of Contents
• Modern Perl Books
• The Perl Philosophy
• Perl and Its Community
• → The Perl Language ←
• Regular Expressions and Matching
• Style and Efficacy
• Managing Real Programs
• Perl Beyond Syntax
• What to Avoid
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The Perl Language

Like a spoken language, the whole of Perl is a combination of several smaller but interrelated parts. Unlike spoken language, where nuance and tone of voice and intuition allow people to communicate despite slight misunderstandings and fuzzy concepts, computers and source code require precision. You can write effective Perl code without knowing every detail of every language feature, but you must understand how they work together to write Perl code well.

Names (or identifiers ) are everywhere in Perl programs: variables, functions, packages, classes, and even filehandles. These names all begin with a letter or an underscore and may optionally include any combination of letters, numbers, and underscores. When the utf8 pragma ( Unicode and Strings ) is in effect, you may use any UTF-8 word characters in identifiers. These are all valid Perl identifiers:

These are invalid Perl identifiers:

Names exist primarily for the benefit of the programmer . These rules apply only to literal names which appear as-is in your source code, such as sub fetch_pie or my $waffleiron . Only Perl's parser enforces the rules about identifier names.

Perl's dynamic nature allows you to refer to entities with names generated at runtime or provided as input to a program. These symbolic lookups provide flexibility at the expense of some safety. In particular, invoking functions or methods indirectly or looking up symbols in a namespace lets you bypass Perl's parser.

Doing so can produce confusing code. As Mark Jason Dominus recommends so effectively http://perl.plover.com/varvarname.html , use a hash ( Hashes ) or nested data structure ( Nested Data Structures ).

Variable Names and Sigils

Variable names always have a leading sigil (or symbol) which indicates the type of the variable's value. Scalar variables ( Scalars ) use the dollar sign ( $ ). Array variables ( Arrays ) use the at sign ( @ ). Hash variables ( Hashes ) use the percent sign ( % ):

These sigils provide a visual namespacing for variable names. It's possible—though confusing—to declare multiple variables of the same name with different types:

Though Perl won't get confused, people reading this code will.

Perl 5's sigils are variant sigils . As context determines how many items you expect from an operation or what type of data you expect to get, so the sigil governs how you manipulate the data of a variable. For example, to access a single element of an array or a hash, you must use the scalar sigil ( $ ):

The parallel with amount context is important. Using a scalar element of an aggregate as an lvalue (the target of an assignment, on the left side of the = character) imposes scalar context ( Context ) on the rvalue (the value assigned, on the right side of the = character).

Similarly, accessing multiple elements of a hash or an array—an operation known as slicing —uses the at symbol ( @ ) and imposes list context ... even if the list itself has zero or one elements :

The most reliable way to determine the type of a variable—scalar, array, or hash—is to look at the operations performed on it. Scalars support all basic operations, such as string, numeric, and boolean manipulations. Arrays support indexed access through square brackets. Hashes support keyed access through curly brackets.

Perl provides a mechanism to group similar functions and variables into their own unique named spaces— namespaces ( Packages ). A namespace is a named collection of symbols. Perl allows multi-level namespaces, with names joined by double colons ( :: ), where DessertShop::IceCream refers to a logical collection of related variables and functions, such as scoop() and pour_hot_fudge() .

Within a namespace, you may use the short name of its members. Outside of the namespace, refer to a member using its fully-qualified name . That is, within DessertShop::IceCream , add_sprinkles() refers to the same function as does DessertShop::IceCream::add_sprinkles() outside of the namespace.

While standard naming rules apply to package names, by convention user-defined packages all start with uppercase letters. The Perl core reserves lowercase package names for core pragmas ( Pragmas ), such as strict and warnings . This is a policy enforced primarily by community guidelines.

All namespaces in Perl 5 are globally visible. When Perl looks up a symbol in DessertShop::IceCream::Freezer , it looks in the main:: symbol table for a symbol representing the DessertShop:: namespace, then in there for the IceCream:: namespace, and so on. The Freezer:: is visible from outside of the IceCream:: namespace. The nesting of the former within the latter is only a storage mechanism, and implies nothing further about relationships between parent and child or sibling packages. Only a programmer can make logical relationships between entities obvious—by choosing good names and organizing them well.

A variable in Perl is a storage location for a value ( Values ). While a trivial program can manipulate values directly, most programs work with variables to simplify the logic of the code. A variable represents values; it's easier to explain the Pythagorean theorem in terms of the variables a , b , and c than by intuiting its principle by producing a long list of valid values. This concept may seem basic, but effective programming requires you to manage the art of balancing the generic and reusable with the specific.

Variable Scopes

Variables are visible to portions of your program depending on their scope ( Scope ). Most of the variables you will encounter have lexical scope ( Lexical Scope ). Files themselves provide their own lexical scopes, such that the package declaration on its own does not create a new scope:

As of Perl 5.14, you may provide a block to the package declaration. This syntax does provide a lexical scope:

Variable Sigils

The sigil of the variable in a declaration determines the type of the variable: scalar, array, or hash. The sigil used when accessing a variable varies depending on what you do to the variable. For example, you declare an array as @values . Access the first element—a single value—of the array with $values[0] . Access a list of values from the array with @values[ @indices ] .

Anonymous Variables

Perl variables do not require names. Names exist to help you, the programmer, keep track of an $apple , @barrels , or %cheap_meals . Variables created without literal names in your source code are anonymous variables. The only way to access anonymous variables is by reference ( References ).

Variables, Types, and Coercion

A variable in Perl 5 represents both a value (a dollar cost, available pizza toppings, guitar shops with phone numbers) and the container which stores that value. Perl's type system deals with value types and container types . While a variable's container type —scalar, array, or hash—cannot change, Perl is flexible about a variable's value type. You may store a string in a variable in one line, append to that variable a number on the next, and reassign a reference to a function ( Function References ) on the third.

Performing an operation on a variable which imposes a specific value type may cause coercion ( Coercion ) from the variable's existing value type.

For example, the documented way to determine the number of entries in an array is to evaluate that array in scalar context ( Context ). Because a scalar variable can only ever contain a scalar, assigning an array to a scalar imposes scalar context on the operation, and an array evaluated in scalar context returns the number of elements in the array:

This relationship between variable types, sigils, and context is essential.

The structure of a program depends heavily on the means by which you model your data with appropriate variables.

Where variables allow the abstract manipulation of data, the values they hold make programs concrete and useful. The more accurate your values, the better your programs. These values are data—your aunt's name and address, the distance between your office and a golf course on the moon, or the weight of all of the cookies you've eaten in the past year. Within your program, the rules regarding the format of that data are often strict. Effective programs need effective (simple, fast, most compact, most efficient) ways of representing their data.

A string is a piece of textual or binary data with no particular formatting or contents. It could be your name, the contents of an image file, or your program itself. A string has meaning in the program only when you give it meaning.

To represent a literal string in your program, surround it with a pair of quoting characters. The most common string delimiters are single and double quotes:

Characters in a single-quoted string represent themselves literally, with two exceptions. Embed a single quote inside a single-quoted string by escaping the quote with a leading backslash:

You must also escape any backslash at the end of the string to avoid escaping the closing delimiter and producing a syntax error:

Any other backslash will be part of the string as it appears, unless two backslashes are adjacent, in which case the first will escape the second:

A double-quoted string has several more special characters available. For example, you may encode otherwise invisible whitespace characters in the string:

This demonstrates a useful principle: the syntax used to declare a string may vary. You can represent a tab within a string with the \t escape or by typing a tab directly. Within Perl's purview, both strings behave the same way, even though the specific representation of the string may differ in the source code.

A string declaration may cross logical newlines; these two declarations are equivalent:

These sequences are often easier to read than their whitespace equivalents.

Perl strings have variable lengths. As you manipulate and modify strings, Perl will change their sizes as appropriate. For example, you can combine multiple strings into a larger string with the concatenation operator . :

This is effectively the same as if you'd initialized the string all at once.

You may also interpolate the value of a scalar variable or the values of an array within a double-quoted string, such that the current contents of the variable become part of the string as if you'd concatenated them:

Include a literal double-quote inside a double-quoted string by escaping it (that is, preceding it with a leading backslash):

When repeated backslashing becomes unwieldy, use an alternate quoting operator by which you can choose an alternate string delimiter. The q operator indicates single quoting, while the qq operator provides double quoting behavior. The character immediately following the operator determines the characters used to delimit the strings. If the character is the opening character of a balanced pair—such as opening and closing braces—the closing character will be the final delimiter. Otherwise, the character itself will be both the starting and ending delimiter.

When declaring a complex string with a series of embedded escapes is tedious, use the heredoc syntax to assign one or more lines of a string:

The <<'END_BLURB' syntax has three parts. The double angle-brackets introduce the heredoc. The quotes determine whether the heredoc obeys single- or double-quoted behavior. The default behavior is double-quoted interpolation. END_BLURB is an arbitrary identifier which the Perl 5 parser uses as the ending delimiter.

Be careful; regardless of the indentation of the heredoc declaration itself, the ending delimiter must start at the beginning of the line:

If the identifier begins with whitespace, that same whitespace must be present before the ending delimiter. Yet if you indent the identifier, Perl 5 will not remove equivalent whitespace from the start of each line of the heredoc.

Using a string in a non-string context will induce coercion ( Coercion ).

Unicode and Strings

Unicode is a system for representing the characters of the world's written languages. While most English text uses a character set of only 127 characters (which requires seven bits of storage and fits nicely into eight-bit bytes), it's naïve to believe that you won't someday need an umlaut.

Perl 5 strings can represent either of two separate but related data types:

Sequences of Unicode characters

Each character has a codepoint , a unique number which identifies it in the Unicode character set.

Sequences of octets

Binary data is a sequence of octets —8 bit numbers, each of which can represent a number between 0 and 255.

Words Matter

Why octet and not byte ? Assuming that one character fits in one byte will cause you no end of Unicode grief. Separate the idea of memory storage from character representation.

Unicode strings and binary strings look similar. Each has a length() . Each supports standard string operations such as concatenation, splicing, and regular expression processing. Any string which is not purely binary data is textual data, and should be a sequence of Unicode characters.

However, because of how your operating system represents data on disk or from users or over the network—as sequences of octets—Perl can't know if the data you read is an image file or a text document or anything else. By default, Perl treats all incoming data as sequences of octets. You must add a specific meaning to that data.

Character Encodings

A Unicode string is a sequence of octets which represents a sequence of characters. A Unicode encoding maps octet sequences to characters. Some encodings, such as UTF-8, can encode all of the characters in the Unicode character set. Other encodings represent a subset of Unicode characters. For example, ASCII encodes plain English text with no accented characters, while Latin-1 can represent text in most languages which use the Latin alphabet.

To avoid most Unicode problems, always decode to and from the appropriate encoding at the inputs and outputs of your program.

An Evolving Standard

Perl 5.12 supports the Unicode 5.2 standard, while Perl 5.14 supports Unicode 6.0. If you need to care about the differences between Unicode versions, you probably already know to see http://unicode.org/versions/ .

Unicode in Your Filehandles

When you tell Perl that a specific filehandle ( Files ) works with encoded text, Perl will convert the incoming octets to Unicode strings automatically. To do this, add an IO layer to the mode of the open builtin. An IO layer wraps around input or output and converts the data. In this case, the :utf8 layer decodes UTF-8 data:

You may also modify an existing filehandle with binmode , whether for input or output:

Without the utf8 mode, printing Unicode strings to a filehandle will result in a warning ( Wide character in %s ), because files contain octets, not Unicode characters.

Unicode in Your Data

The core module Encode provides a function named decode() to convert a scalar containing data to a Unicode string. The corresponding encode() function converts from Perl's internal encoding to the desired output encoding:

Unicode in Your Programs

You may include Unicode characters in your programs in three ways. The easiest is to use the utf8 pragma ( Pragmas ), which tells the Perl parser to interpret the rest of the source code file with the UTF-8 encoding. This allows you to use Unicode characters in strings and identifiers:

To write this code, your text editor must understand UTF-8 and you must save the file with the appropriate encoding.

Within double-quoted strings, you may use the Unicode escape sequence to represent character encodings. The syntax \x{} represents a single character; place the hex form of the character's Unicode number within the curly brackets:

Some Unicode characters have names, and these names are often clearer to read than Unicode numbers. Use the charnames pragma to enable them and the \N{} escape to refer to them:

You may use the \x{} and \N{} forms within regular expressions as well as anywhere else you may legitimately use a string or a character.

Implicit Conversion

Most Unicode problems in Perl arise from the fact that a string could be either a sequence of octets or a sequence of characters. Perl allows you to combine these types through the use of implicit conversions. When these conversions are wrong, they're rarely obviously wrong.

When Perl concatenates a sequences of octets with a sequence of Unicode characters, it implicitly decodes the octet sequence using the Latin-1 encoding. The resulting string will contain Unicode characters. When you print Unicode characters, Perl will encode the string using UTF-8, because Latin-1 cannot represent the entire set of Unicode characters—Latin-1 is a subset of UTF-8.

This asymmetry can lead to Unicode strings encoded as UTF-8 for output and decoded as Latin-1 when input.

Worse yet, when the text contains only English characters with no accents, the bug hides—because both encodings have the same representation for every character.

If $name contains an English name such as Alice you will never notice any problem, because the Latin-1 representation is the same as the UTF-8 representation. If $name contains a name such as José , $name can contain several possible values:

• $name contains four Unicode characters.
• $name contains four Latin-1 octets representing four Unicode characters.
• $name contains five UTF-8 octets representing four Unicode characters.

The string literal has several possible scenarios:

• It is an ASCII string literal and contains octets. my $hello = "Hello, ";

The string literal contains octets.

• It is a non-ASCII string literal with the utf8 or encoding pragma in effect and contains Unicode characters. use utf8; my $hello = "Kuirabá, ";

If both $hello and $name are Unicode strings, the concatenation will produce another Unicode string.

If both strings are octet streams, Perl will concatenate them into a new octet string. If both values are octets of the same encoding—both Latin-1, for example, the concatenation will work correctly. If the octets do not share an encoding, for example a concatenation appending UTF-8 data to Latin-1 data, then the resulting sequence of octets makes sense in neither encoding. This could happen if the user entered a name as UTF-8 data and the greeting were a Latin-1 string literal, but the program decoded neither.

If only one of the values is a Unicode string, Perl will decode the other as Latin-1 data. If this is not the correct encoding, the resulting Unicode characters will be wrong. For example, if the user input were UTF-8 data and the string literal were a Unicode string, the name would be incorrectly decoded into five Unicode characters to form José ( sic ) instead of José because the UTF-8 data means something else when decoded as Latin-1 data.

See perldoc perluniintro for a far more detailed explanation of Unicode, encodings, and how to manage incoming and outgoing data in a Unicode world For far more detail about managing Unicode effectively throughout your programs, see Tom Christiansen's answer to "Why does Modern Perl avoid UTF-8 by default?" http://stackoverflow.com/questions/6162484/why-does-modern-perl-avoid-utf-8-by-default/6163129#6163129 .

Perl 5.12 added a feature, unicode_strings , which enables Unicode semantics for all string operations within its scope. Perl 5.14 improved this feature; if you work with Unicode in Perl, it's worth upgrading to at least Perl 5.14.

Perl supports numbers as both integers and floating-point values. You may represent them with scientific notation as well as in binary, octal, and hexadecimal forms:

The emboldened characters are the numeric prefixes for binary, octal, and hex notation respectively. Be aware that a leading zero on an integer always indicates octal mode.

When 1.99 + 1.99 is 4

Even though you can write floating-point values explicitly in Perl 5 with perfect accuracy, Perl 5 stores them internally in a binary format. This representation is sometimes imprecise in specific ways; consult perldoc perlnumber for more details.

You may not use commas to separate thousands in numeric literals, lest the parser interpret the commas as comma operators. Instead, use underscores within the number. The parser will treat them as invisible characters; your readers may not. These are equivalent:

Consider the most readable alternative.

Because of coercion ( Coercion ), Perl programmers rarely have to worry about converting text read from outside the program to numbers. Perl will treat anything which looks like a number as a number in numeric contexts. In the rare circumstances where you need to know if something looks like a number to Perl, use the looks_like_number function from the core module Scalar::Util . This function returns a true value if Perl will consider the given argument numeric.

The Regexp::Common module from the CPAN provides several well-tested regular expressions to identify more specific valid types (whole number, integer, floating-point value) of numeric values.

Perl 5's undef value represents an unassigned, undefined, and unknown value. Declared but undefined scalar variables contain undef :

undef evaluates to false in boolean context. Evaluating undef in a string context—such as interpolating it into a string—produces an uninitialized value warning:

... produces:

The defined builtin returns a true value if its operand evaluates to a defined value (anything other than undef ):

The Empty List

When used on the right-hand side of an assignment, the () construct represents an empty list. In scalar context, this evaluates to undef . In list context, it is an empty list. When used on the left-hand side of an assignment, the () construct imposes list context. To count the number of elements returned from an expression in list context without using a temporary variable, use the idiom ( Idioms ):

Because of the right associativity ( Associativity ) of the assignment operator, Perl first evaluates the second assignment by calling get_all_clown_hats() in list context. This produces a list.

Assignment to the empty list throws away all of the values of the list, but that assignment takes place in scalar context, which evaluates to the number of items on the right hand side of the assignment. As a result, $count contains the number of elements in the list returned from get_all_clown_hats() .

If you find that concept confusing right now, fear not. As you understand how Perl's fundamental design features fit together in practice, it will make more sense.

A list is a comma-separated group of one or more expressions. Lists may occur verbatim in source code as values:

... as targets of assignments:

... or as lists of expressions:

Parentheses do not create lists. The comma operator creates lists. Where present, the parentheses in these examples group expressions to change their precedence ( Precedence ).

Use the range operator to create lists of literals in a compact form:

Use the qw() operator to split a literal string on whitespace to produce a list of strings:

No Comment Please

Perl will emit a warning if a qw() contains a comma or the comment character ( # ), because not only are such characters rare in a qw() , their presence usually indicates an oversight.

Lists can (and often do) occur as the results of expressions, but these lists do not appear literally in source code.

Lists and arrays are not interchangeable in Perl. Lists are values. Arrays are containers. You may store a list in an array and you may coerce an array to a list, but they are separate entities. For example, indexing into a list always occurs in list context. Indexing into an array can occur in scalar context (for a single element) or list context (for a slice):

Control Flow

Perl's basic control flow is straightforward. Program execution starts at the beginning (the first line of the file executed) and continues to the end:

Perl's control flow directives change the order of execution—what happens next in the program—depending on the values of their expressions.

Branching Directives

The if directive performs the associated action only when its conditional expression evaluates to a true value:

This postfix form is useful for simple expressions. A block form groups multiple expressions into a single unit:

While the block form requires parentheses around its condition, the postfix form does not.

The conditional expression may consist of multiple subexpressions, as long as it evaluates to a single top-level expression:

In the postfix form, adding parentheses can clarify the intent of the code at the expense of visual cleanliness:

The unless directive is a negated form of if . Perl will perform the action when the conditional expression evaluates to false :

Like if , unless also has a block form, though many programmers avoid it, as it rapidly becomes difficult to read with complex conditionals:

unless works very well for postfix conditionals, especially parameter validation in functions ( Postfix Parameter Validation ):

The block forms of if and unless both work with the else directive, which provides code to run when the conditional expression does not evaluate to true (for if ) or false (for unless ):

else blocks allow you to rewrite if and unless conditionals in terms of each other:

However, the implied double negative of using unless with an else block can be confusing. This example may be the only place you ever see it.

Just as Perl provides both if and unless to allow you to phrase your conditionals in the most readable way, you can choose between positive and negative conditional operators:

... though the double negative implied by the presence of the else block suggests inverting the conditional.

One or more elsif directives may follow an if block form and may precede any single else :

An unless chain may also use an elsif block Good luck deciphering that! . There is no elseunless .

Writing else if is a syntax error Larry prefers elsif for aesthetic reasons, as well the prior art of the Ada programming language. :

The Ternary Conditional Operator

The ternary conditional operator evaluates a conditional expression and produces one of two alternatives:

The conditional expression precedes the question mark character ( ? ) and the colon character ( : ) separates the alternatives. The alternatives are expressions of arbitrary complexity—including other ternary conditional expressions.

An interesting, though obscure, idiom is to use the ternary conditional to select between alternative variables , not only values:

Again, weigh the benefits of clarity versus the benefits of conciseness.

Short Circuiting

Perl exhibits short-circuiting behavior when it encounters complex conditional expressions. When Perl can determine that a complex expression would succeed or fail as a whole without evaluating every subexpression, it will not evaluate subsequent subexpressions. This is most obvious with an example:

The return value of ok() ( Testing ) is the boolean value obtained by evaluating the first argument, so this code prints:

When the first subexpression—the first call to ok —evaluates to a true value, Perl must evaluate the second subexpression. If the first subexpression had evaluated to a false value, there would be no need to check subsequent subexpressions, as the entire expression could not succeed:

This example prints:

Even though the second subexpression would obviously succeed, Perl never evaluates it. The same short-circuiting behavior is evident for logical-or operations:

With the success of the first subexpression, Perl can avoid evaluating the second subexpression. If the first subexpression were false, the result of evaluating the second subexpression would dictate the result of evaluating the entire expression.

Besides allowing you to avoid potentially expensive computations, short circuiting can help you to avoid errors and warnings, as in the case where using an undefined value might raise a warning:

Context for Conditional Directives

The conditional directives— if , unless , and the ternary conditional operator—all evaluate an expression in boolean context ( Context ). As comparison operators such as eq , == , ne , and != all produce boolean results when evaluated, Perl coerces the results of other expressions—including variables and values—into boolean forms.

Perl 5 has no single true value, nor a single false value. Any number which evaluates to 0 is false. This includes 0 , 0.0 , 0e0 , 0x0 , and so on. The empty string ( '' ) and '0' evaluate to a false value, but the strings '0.0' , '0e0' , and so on do not. The idiom '0 but true' evaluates to 0 in numeric context but true in boolean context, thanks to its string contents.

Both the empty list and undef evaluate to a false value. Empty arrays and hashes return the number 0 in scalar context, so they evaluate to a false value in boolean context. An array which contains a single element—even undef —evaluates to true in boolean context. A hash which contains any elements—even a key and a value of undef —evaluates to a true value in boolean context.

Greater Control Over Context

The Want module from the CPAN allows you to detect boolean context within your own functions. The core overloading pragma ( Overloading ) allows you to specify what your own data types produce when evaluated in various contexts.

Looping Directives

Perl provides several directives for looping and iteration. The foreach -style loop evaluates an expression which produces a list and executes a statement or block until it has consumed that list:

This example uses the range operator to produce a list of integers from one to ten inclusive. The foreach directive loops over them, setting the topic variable $_ ( The Default Scalar Variable ) to each in turn. Perl executes the block for each integer and prints the squares of the integers.

foreach versus for

Many Perl programmers refer to iteration as foreach loops, but Perl treats the names foreach and for interchangeably. The subsequent code determines the type and behavior of the loop.

Like if and unless , this loop has a postfix form:

A for loop may use a named variable instead of the topic:

When a for loop uses an iterator variable, the variable scope is within the loop. Perl will set this lexical to the value of each item in the iteration. Perl will not modify the topic variable ( $_ ). If you have declared a lexical $i in an outer scope, its value will persist outside the loop:

This localization occurs even if you do not redeclare the iteration variable as a lexical ... but do declare your iteration variables as lexicals to reduce their scope. :

Iteration and Aliasing

The for loop aliases the iterator variable to the values in the iteration such that any modifications to the value of the iterator modifies the iterated value in place:

This aliasing also works with the block style for loop:

... as well as iteration with the topic variable:

You cannot use aliasing to modify constant values, however:

Instead Perl will produce an exception about modification of read-only values.

You may occasionally see the use of for with a single scalar variable to alias $_ to the variable:

Iteration and Scoping

Iterator scoping with the topic variable provides one common source of confusion. Consider a function topic_mangler() which modifies $_ on purpose. If code iterating over a list called topic_mangler() without protecting $_ , debugging fun would ensue:

If you must use $_ rather than a named variable, make the topic variable lexical with my $_ :

Using a named iteration variable also prevents undesired aliasing behavior through $_ .

The C-Style For Loop

The C-style for loop requires you to manage the conditions of iteration:

You must explicitly assign to an iteration variable in the looping construct, as this loop performs neither aliasing nor assignment to the topic variable. While any variable declared in the loop construct is scoped to the lexical block of the loop, there is no lexicalization of a variable declared outside of the loop construct:

The looping construct may have three subexpressions. The first subexpression—the initialization section—executes only once, before the loop body executes. Perl evaluates the second subexpression—the conditional comparison—before each iteration of the loop body. When this evaluates to a true value, iteration proceeds. When it evaluates to a false value, iteration stops. The final subexpression executes after each iteration of the loop body.

Note the lack of a semicolon after the final subexpression as well as the use of the comma operator and low-precedence and ; this syntax is surprisingly finicky. When possible, prefer the foreach -style loop to the for loop.

All three subexpressions are optional. An infinite for loop might be:

While and Until

A while loop continues until the loop conditional expression evaluates to a boolean false value. An idiomatic infinite loop is:

Unlike the iteration foreach -style loop, the while loop's condition has no side effects by itself. That is, if @values has one or more elements, this code is also an infinite loop:

To prevent such an infinite while loop, use a destructive update of the @values array by modifying the array with each loop iteration:

Modifying @values inside of the while condition check also works, but it has some subtleties related to the truthiness of each value.

This loop will exit as soon as it reaches an element that evaluates to a false value, not necessarily when it has exhausted the array. That may be the desired behavior, but is often surprising to novices.

The until loop reverses the sense of the test of the while loop. Iteration continues while the loop conditional expression evaluates to a false value:

The canonical use of the while loop is to iterate over input from a filehandle:

Perl 5 interprets this while loop as if you had written:

Without the implicit defined , any line read from the filehandle which evaluated to a false value in a scalar context—a blank line or a line which contained only the character 0 —would end the loop. The readline ( <> ) operator returns an undefined value only when it has reached the end of the file.

chomp Your Lines

Use the chomp builtin to remove line-ending characters from each line. Many novices forget this.

Both while and until have postfix forms, such as the infinite loop 1 while 1; . Any single expression is suitable for a postfix while or until , including the classic "Hello, world!" example from 8-bit computers of the early 1980s:

Infinite loops are more useful than they seem, especially for event loops in GUI programs, program interpreters, or network servers:

Use a do block to group several expressions into a single unit:

A do block parses as a single expression which may contain several expressions. Unlike the while loop's block form, the do block with a postfix while or until will execute its body at least once. This construct is less common than the other loop forms, but no less powerful.

Loops within Loops

You may nest loops within other loops:

When you do so, declare named iteration variables! The potential for confusion with the topic variable and its scope is too great otherwise.

A common mistake with nesting foreach and while loops is that it is easy to exhaust a filehandle with a while loop:

Opening the filehandle outside of the for loop leaves the file position unchanged between each iteration of the for loop. On its second iteration, the while loop will have nothing to read and will not execute. To solve this problem, re-open the file inside the for loop (simple to understand, but not always a good use of system resources), slurp the entire file into memory (which may not work if the file is large), or seek the filehandle back to the beginning of the file for each iteration (an often overlooked option):

Loop Control

Sometimes you need to break out of a loop before you have exhausted the iteration conditions. Perl 5's standard control mechanisms—exceptions and return —work, but you may also use loop control statements.

The next statement restarts the loop at its next iteration. Use it when you've done all you need to in the current iteration. To loop over lines in a file but skip everything that starts with the comment character # , write:

Multiple Exits versus Nested Ifs

Compare the use of next with the alternative: wrapping the rest of the body of the block in an if . Now consider what happens if you have multiple conditions which could cause you to skip a line. Loop control modifiers with postfix conditionals can make your code much more readable.

The last statement ends the loop immediately. To finish processing a file once you've seen the ending token, write:

The redo statement restarts the current iteration without evaluating the conditional again. This can be useful in those few cases where you want to modify the line you've read in place, then start processing over from the beginning without clobbering it with another line. To implement a silly file parser that joins lines which end with a backslash:

Using loop control statements in nested loops can be confusing. If you cannot avoid nested loops—by extracting inner loops into named functions—use a loop label to clarify:

The continue construct behaves like the third subexpression of a for loop; Perl executes its block before subsequent iterations of a loop, whether due to normal loop repetition or premature re-iteration from next The Perl equivalent to C's continue is next . . You may use it with a while , until , when , or for loop. Examples of continue are rare, but it's useful any time you want to guarantee that something occurs with every iteration of the loop regardless of how that iteration ends:

Be aware that a continue block does not execute when control flow leaves a loop due to last or redo .

The given construct is a feature new to Perl 5.10. It assigns the value of an expression to the topic variable and introduces a block:

Unlike for , it does not iterate over an aggregate. It evaluates its expression in scalar context, and always assigns to the topic variable:

given also lexicalizes the topic variable:

given is most useful when combined with when ( Smart Matching ). given topicalizes a value within a block so that multiple when statements can match the topic against expressions using smart-match semantics. To write the Rock, Paper, Scissors game:

Perl executes the default rule when none of the other conditions match.

Simplified Dispatch with Multimethods

The CPAN module MooseX::MultiMethods provides another technique to simplify this code.

A tailcall occurs when the last expression within a function is a call to another function—the outer function's return value is the inner function's return value:

Returning from greet_person() directly to the caller of log_and_greet_person() is more efficient than returning to log_and_greet_person() and immediately from log_and_greet_person() . Returning directly from greet_person() to the caller of log_and_greet_person() is a tailcall optimization .

Heavily recursive code ( Recursion ), especially mutually recursive code, can consume a lot of memory. Tailcalls reduce the memory needed for internal bookkeeping of control flow and can make expensive algorithms tractable. Unfortunately, Perl 5 does not automatically perform this optimization; you have to do it yourself when it's necessary.

The builtin goto operator has a form which calls a function as if the current function were never called, essentially erasing the bookkeeping for the new function call. The ugly syntax confuses people who've heard "Never use goto ", but it works:

This example has two important features. First, goto &function_name or goto &$function_reference requires the use of the function sigil ( & ) so that the parser knows to perform a tailcall instead of jumping to a label. Second, this form of function call passes the contents of @_ implicitly to the called function. You may modify @_ to change the passed arguments.

This technique is relatively rare; it's most useful when you want to hijack control flow to get out of the way of other functions inspecting caller (such as when you're implementing special logging or some sort of debugging feature), or when using an algorithm which requires a lot of recursion.

Perl 5's fundamental data type is the scalar , a single, discrete value. That value may be a string, an integer, a floating point value, a filehandle, or a reference—but it is always a single value. Scalars may be lexical, package, or global ( Global Variables ) variables. You may only declare lexical or package variables. The names of scalar variables must conform to standard variable naming guidelines ( Names ). Scalar variables always use the leading dollar-sign ( $ ) sigil ( Variable Sigils ).

Variant Sigils and Context

Scalar values and scalar context have a deep connection; assigning to a scalar provides scalar context. Using the scalar sigil with an aggregate variable imposes scalar context to access a single element of the hash or array.

Scalars and Types

A scalar variable can contain any type of scalar value without special conversions or casts, and the type of value stored in a variable can change:

Even though this code is legal , changing the type of data stored in a scalar is a sign of confusion.

This flexibility of type often leads to value coercion ( Coercion ). For example, you may treat the contents of a scalar as a string, even if you didn't explicitly assign it a string:

You may also use mathematical operations on strings:

One-Way Increment Magic

This magical string increment behavior has no corresponding magical decrement behavior. You can't get the previous string value by writing $call_sign-- .

This string increment operation turns a into b and z into aa , respecting character set and case. While ZZ9 becomes AAA0 , ZZ09 becomes ZZ10 —numbers wrap around while there are more significant places to increment, as on a vehicle odometer.

Evaluating a reference ( References ) in string context produces a string. Evaluating a reference in numeric context produces a number. Neither operation modifies the reference in place, but you cannot recreate the reference from either result:

$authors is still useful as a reference, but $stringy_ref is a string with no connection to the reference and $numeric_ref is a number with no connection to the reference.

To allow coercion without data loss, Perl 5 scalars can contain both numeric and string components. The internal data structure which represents a scalar in Perl 5 has a numeric slot and a string slot. Accessing a string in a numeric context produces a scalar with both string and numeric values. The dualvar() function within the core Scalar::Util module allows you to manipulate both values directly within a single scalar.

Scalars do not contain a separate slot for boolean values. In boolean context, the empty string ( '' ) and '0' are false. All other strings are true. In boolean context, numbers which evaluate to zero ( 0 , 0.0 , and 0e0 ) are false. All other numbers are true.

What is Truth?

Be careful that the strings '0.0' and '0e0' are true; this is one place where Perl 5 makes a distinction between what looks like a number and what really is a number.

One other value is always false: undef . This is the value of uninitialized variables as well as a value in its own right.

Perl 5 arrays are first-class data structures—the language supports them as a built-in data type—which store zero or more scalars. You can access individual members of the array by integer indexes, and you can add or remove elements at will. The @ sigil denotes an array. To declare an array:

Array Elements

Accessing an individual element of an array in Perl 5 requires the scalar sigil. $cats[0] is an unambiguous use of the @cats array, because postfix ( Fixity ) square brackets ( [] ) always mean indexed access to an array.

The first element of an array is at index zero:

The last index of an array depends on the number of elements in the array. An array in scalar context (due to scalar assignment, string concatenation, addition, or boolean context) evaluates to the number of elements in the array:

To get the index of the final element of an array, subtract one from the number of elements of the array (remember that array indexes start at 0) or use the unwieldy $#cats syntax:

When the index matters less than the position of an element, use negative array indices instead. The last element of an array is available at the index -1 . The second to last element of the array is available at index -2 , and so on:

$# has another use: resize an array in place by assigning to it. Remember that Perl 5 arrays are mutable. They expand or contract as necessary. When you shrink an array, Perl will discard values which do not fit in the resized array. When you expand an array, Perl will fill the expanded positions with undef .

Array Assignment

Assign to individual positions in an array directly by index:

If you assign to an index beyond the array's current bound, Perl will extend the array to account for the new size and will fill in all intermediary positions with undef . After the first assignment, the array will contain undef at positions 0, 1, and 2 and Jack at position 3.

As an assignment shortcut, initialize an array from a list:

... but remember that these parentheses do not create a list. Without parentheses, this would assign Daisy as the first and only element of the array, due to operator precedence ( Precedence ).

Any expression which produces a list in list context can assign to an array:

Assigning to a scalar element of an array imposes scalar context, while assigning to the array as a whole imposes list context.

To clear an array, assign an empty list:

Arrays Start Empty

my @items = (); is a longer and noisier version of my @items because freshly-declared arrays start out empty.

Array Operations

Sometimes an array is more convenient as an ordered, mutable collection of items than as a mapping of indices to values. Perl 5 provides several operations to manipulate array elements without using indices.

The push and pop operators add and remove elements from the tail of an array, respectively:

You may push a list of values onto an array, but you may only pop one at a time. push returns the new number of elements in the array. pop returns the removed element.

Because push operates on a list, you can easily append the elements of one or more arrays to another with:

Similarly, unshift and shift add elements to and remove an element from the start of an array, respectively:

unshift prepends a list of elements to the start of the array and returns the new number of elements in the array. shift removes and returns the first element of the array.

Few programs use the return values of push and unshift .

The splice operator removes and replaces elements from an array given an offset, a length of a list slice, and replacements. Both replacing and removing are optional; you may omit either behavior. The perlfunc description of splice demonstrates its equivalences with push , pop , shift , and unshift . One effective use is removal of two elements from an array:

Prior to Perl 5.12, iterating over an array by index required a C-style loop. As of Perl 5.12, each can iterate over an array by index and value:

Array Slices

The array slice construct allows you to access elements of an array in list context. Unlike scalar access of an array element, this indexing operation takes a list of zero or more indices and uses the array sigil ( @ ):

Array slices are useful for assignment:

A slice can contain zero or more elements—including one:

The only syntactic difference between an array slice of one element and the scalar access of an array element is the leading sigil. The semantic difference is greater: an array slice always imposes list context. An array slice evaluated in scalar context will produce a warning:

An array slice imposes list context on the expression used as its index:

Arrays and Context

In list context, arrays flatten into lists. If you pass multiple arrays to a normal Perl 5 function, they will flatten into a single list:

Within the function, @_ will contain seven elements, not two, because list assignment to arrays is greedy . An array will consume as many elements from the list as possible. After the assignment, @cats will contain every argument passed to the function. @dogs will be empty.

This flattening behavior sometimes confuses novices who attempt to create nested arrays in Perl 5:

... but this code is effectively the same as:

... because these parentheses merely group expressions. They do not create lists in these circumstances. To avoid this flattening behavior, use array references ( Array References ).

Array Interpolation

Arrays interpolate in strings as lists of the stringifications of each item separated by the current value of the magic global $" . The default value of this variable is a single space. Its English.pm mnemonic is $LIST_SEPARATOR . Thus:

Localize $" with a delimiter to ease your debugging Credit goes to Mark Jason Dominus for this technique. :

A hash is a first-class Perl data structure which associates string keys with scalar values. In the same way that the name of a variable corresponds to a storage location, a key in a hash refers to a value. Think of a hash like you would a telephone book: use the names of your friends to look up their numbers. Other languages call hashes tables , associative arrays , dictionaries , or maps .

Hashes have two important properties: they store one scalar per unique key and they provide no specific ordering of keys.

Declaring Hashes

Hashes use the % sigil. Declare a lexical hash with:

A hash starts out empty. You could write my %favorite_flavors = (); , but that's redundant.

Hashes use the scalar sigil $ when accessing individual elements and curly braces { } for keyed access:

Assign a list of keys and values to a hash in a single expression:

If you assign an odd number of elements to the hash, you will receive a warning to that effect. Idiomatic Perl often uses the fat comma operator ( => ) to associate values with keys, as it makes the pairing more visible:

The fat comma operator acts like the regular comma, but also automatically quotes the previous bareword ( Barewords ). The strict pragma will not warn about such a bareword—and if you have a function with the same name as a hash key, the fat comma will not call the function:

The key of this hash will be name and not Leonardo . To call the function, make the function call explicit:

Assign an empty list to empty a hash You may occasionally see undef %hash . :

Hash Indexing

Access individual hash values with an indexing operation. Use a key (a keyed access operation) to retrieve a value from a hash:

In this example, $name contains a string which is also a key of the hash. As with accessing an individual element of an array, the hash's sigil has changed from % to $ to indicate keyed access to a scalar value.

You may also use string literals as hash keys. Perl quotes barewords automatically according to the same rules as fat commas:

Don't Quote Me

Novices often always quote string literal hash keys, but experienced developers elide the quotes whenever possible. In this way, the presence of quotes in hash keys signifies an intention to do something different.

Even Perl 5 builtins get the autoquoting treatment:

The unary plus ( Unary Coercions ) turns what would be a bareword ( shift ) subject to autoquoting rules into an expression. As this implies, you can use an arbitrary expression—not only a function call—as the key of a hash:

Hash keys can only be strings. Anything that evaluates to a string is an acceptable hash key. Perl will go so far as to coerce ( Coercion ) any non-string into a string, such that if you use an object as a hash key, you'll get the stringified version of that object instead of the object itself:

Hash Key Existence

The exists operator returns a boolean value to indicate whether a hash contains the given key:

Using exists instead of accessing the hash key directly avoids two problems. First, it does not check the boolean nature of the hash value ; a hash key may exist with a value even if that value evaluates to a boolean false (including undef ):

Second, exists avoids autovivification ( Autovivification ) within nested data structures ( Nested Data Structures ).

If a hash key exists, its value may be undef . Check that with defined :

Accessing Hash Keys and Values

Hashes are aggregate variables, but their pairwise nature offers many more possibilities for iteration: over the keys of a hash, the values of a hash, or pairs of keys and values. The keys operator produces a list of hash keys:

The values operator produces a list of hash values:

The each operator produces a list of two-element lists of the key and the value:

Unlike arrays, there is no obvious ordering to these lists. The ordering depends on the internal implementation of the hash, the particular version of Perl you are using, the size of the hash, and a random factor. Even so, the order of hash items is consistent between keys , values , and each . Modifying the hash may change the order, but you can rely on that order if the hash remains the same.

Each hash has only a single iterator for the each operator. You cannot reliably iterate over a hash with each more than once; if you begin a new iteration while another is in progress, the former will end prematurely and the latter will begin partway through the hash. During such iteration, beware not to call any function which may itself try to iterate over the hash with each .

In practice this occurs rarely, but reset a hash's iterator with keys or values in void context when you need it:

Hash Slices

A hash slice is a list of keys or values of a hash indexed in a single operation. To initialize multiple elements of a hash at once:

This is equivalent to the initialization:

... except that the hash slice initialization does not replace the existing contents of the hash.

Hash slices also allow you to retrieve multiple values from a hash in a single operation. As with array slices, the sigil of the hash changes to indicate list context. The use of the curly braces indicates keyed access and makes the hash unambiguous:

Hash slices make it easy to merge two hashes:

This is equivalent to looping over the contents of %canada_addresses manually, but is much shorter.

What if the same key occurs in both hashes? The hash slice approach always overwrites existing key/value pairs in %addresses . If you want other behavior, looping is more appropriate.

The Empty Hash

An empty hash contains no keys or values. It evaluates to a false value in a boolean context. A hash which contains at least one key/value pair evaluates to a true value in boolean context even if all of the keys or all of the values or both would themselves evaluate to false values in a boolean context.

In scalar context, a hash evaluates to a string which represents the ratio of full buckets in the hash—internal details about the hash implementation that you can safely ignore.

In list context, a hash evaluates to a list of key/value pairs similar to what you receive from the each operator. However, you cannot iterate over this list the same way you can iterate over the list produced by each , lest the loop will never terminate:

You can loop over the list of keys and values with a for loop, but the iterator variable will get a key on one iteration and its value on the next, because Perl will flatten the hash into a single list of interleaved keys and values.

Hash Idioms

Because each key exists only once in a hash, assigning the same key to a hash multiple times stores only the most recent key. Use this to find unique list elements:

Using undef with a hash slice sets the values of the hash to undef . This idiom is the cheapest way to perform set operations with a hash.

Hashes are also useful for counting elements, such as IP addresses in a log file:

The initial value of a hash value is undef . The postincrement operator ( ++ ) treats that as zero. This in-place modification of the value increments an existing value for that key. If no value exists for that key, Perl creates a value ( undef ) and immediately increments it to one, as the numification of undef produces the value 0.

This strategy provides a useful caching mechanism to store the result of an expensive operation with little overhead:

This orcish maneuver Or-cache, if you like puns. returns the value from the hash, if it exists. Otherwise, it calculates, caches, and returns the value. The defined-or assignment operator ( //= ) evaluates its left operand. If that operand is not defined, the operator assigns the lvalue the value of its right operand. In other words, if there's no value in the hash for the given key, this function will call create_user() with the key and update the hash.

Perl 5.10 introduced the defined-or and defined-or assignment operators. Prior to 5.10, most code used the boolean-or assignment operator ( ||= ) for this purpose. Unfortunately, some valid values evaluate to a false value in boolean context, so evaluating the definedness of values is almost always more accurate. This lazy orcish maneuver tests for the definedness of the cached value, not truthiness.

If your function takes several arguments, use a slurpy hash ( Slurping ) to gather key/value pairs into a single hash as named function arguments:

This approach allows you to set default values:

... or include them in the hash initialization, as latter assignments take precedence over earlier assignments:

Locking Hashes

As hash keys are barewords, they offer little typo protection compared to the function and variable name protection offered by the strict pragma. The little-used core module Hash::Util provides mechanisms to ameliorate this.

To prevent someone from accidentally adding a hash key you did not intend (whether as a typo or from untrusted user input), use the lock_keys() function to restrict the hash to its current set of keys. Any attempt to add a new key to the hash will raise an exception. This is lax security suitable only for preventing accidents; anyone can use the unlock_keys() function to remove this protection.

Similarly you can lock or unlock the existing value for a given key in the hash ( lock_value() and unlock_value() ) and make or unmake the entire hash read-only with lock_hash() and unlock_hash() .

A Perl variable can hold at various times values of different types—strings, integers, rational numbers, and more. Rather than attaching type information to variables, Perl relies on the context provided by operators ( Numeric, String, and Boolean Context ) to know what to do with values. By design, Perl attempts to do what you mean Called DWIM for do what I mean or dwimmery . , though you must be specific about your intentions. If you treat a variable which happens to contain a number as a string, Perl will do its best to coerce that number into a string.

Boolean Coercion

Boolean coercion occurs when you test the truthiness of a value, such as in an if or while condition. Numeric 0, undef , the empty string, and the string '0' all evaluate as false. All other values—including strings which may be numerically equal to zero (such as '0.0' , '0e' , and '0 but true' )—evaluate as true.

When a scalar has both string and numeric components ( Dualvars ), Perl 5 prefers to check the string component for boolean truth. '0 but true' evaluates to zero numerically, but it is not an empty string, thus it evaluates to a true value in boolean context.

String Coercion

String coercion occurs when using string operators such as comparisons ( eq and cmp ), concatenation, split , substr , and regular expressions, as well as when using a value as a hash key. The undefined value stringifies to an empty string, produces a "use of uninitialized value" warning. Numbers stringify to strings containing their values, such that the value 10 stringifies to the string 10 . You can even split a number into individual digits with:

Numeric Coercion

Numeric coercion occurs when using numeric comparison operators (such as == and <=> ), when performing mathematic operations, and when using a value as an array or list index. The undefined value numifies to zero and produces a "Use of uninitialized value" warning. Strings which do not begin with numeric portions also numify to zero and produce an "Argument isn't numeric" warning. Strings which begin with characters allowed in numeric literals numify to those values and produce no warnings, such that 10 leptons leaping numifies to 10 and 6.022e23 moles marauding numifies to 6.022e23 .

The core module Scalar::Util contains a looks_like_number() function which uses the same parsing rules as the Perl 5 grammar to extract a number from a string.

Mathematicians Rejoice

The strings Inf and Infinity represent the infinite value and behave as numbers. The string NaN represents the concept "not a number". Numifying them produces no "Argument isn't numeric" warning.

Reference Coercion

Using a dereferencing operation on a non-reference turns that value into a reference. This process of autovivification ( Autovivification ) is handy when manipulating nested data structures ( Nested Data Structures ):

Although the hash never contained values for Brad and Jack , Perl helpfully created hash references for them, then assigned each a key/value pair keyed on id .

Cached Coercions

Perl 5's internal representation of values stores both string and numeric values. Stringifying a numeric value does not replace the numeric value. Instead, it attaches a stringified value, so that the representation contains both components. Similarly, numifying a string value populates the numeric component while leaving the string component untouched.

Certain Perl operations prefer to use one component of a value over another—boolean checks prefer strings, for example. If a value has a cached representation in a form you do not expect, relying on an implicit conversion may produce surprising results. You almost never need to be explicit about what you expect Your author can recall doing so twice in over a decade of programming Perl 5 , but knowing that this caching occurs may someday help you diagnose an odd situation.

The multi-component nature of Perl values is available to users in the form of dualvars . The core module Scalar::Util provides a function dualvar() which allows you to bypass Perl coercion and manipulate the string and numeric components of a value separately:

A Perl namespace associates and encapsulates various named entities within a named category, like your family name or a brand name. Unlike a real-world name, a namespace implies no direct relationship between entities. Such relationships may exist, but do not have to.

A package in Perl 5 is a collection of code in a single namespace. The distinction is subtle: the package represents the source code and the namespace represents the entity created when Perl parses that code.

The package builtin declares a package and a namespace:

All global variables and functions declared or referred to after the package declaration refer to symbols within the MyCode namespace. You can refer to the @boxes variable from the main namespace only by its fully qualified name of @MyCode::boxes . A fully qualified name includes a complete package name, so you can call the add_box() function only by MyCode::add_box() .

The scope of a package continues until the next package declaration or the end of the file, whichever comes first. Perl 5.14 enhanced package so that you may provide a block which explicitly delineates the scope of the declaration:

The default package is the main package. Without a package declaration, the current package is main . This rule applies to one-liners, standalone programs, and even .pm files.

Besides a name, a package has a version and three implicit methods, import() ( Importing ), unimport() , and VERSION() . VERSION() returns the package's version number. This number is a series of numbers contained in a package global named $VERSION . By rough convention, versions tend to be a series of integers separated by dots, as in 1.23 or 1.1.10 , where each segment is an integer.

Perl 5.12 introduced a new syntax intended to simplify version numbers, as documented in perldoc version::Internals . These stricter version numbers must have a leading v character and at least three integer components separated by periods:

With Perl 5.14, the optional block form of a package declaration is:

In 5.10 and earlier, the simplest way to declare the version of a package is:

Every package inherits a VERSION() method from the UNIVERSAL base class. You may override VERSION() , though there are few reasons to do so. This method returns the value of $VERSION :

If you provide a version number as an argument, this method will throw an exception unless the version of the module is equal to or greater than the argument:

Packages and Namespaces

Every package declaration creates a new namespace if necessary and causes the parser to put all subsequent package global symbols (global variables and functions) into that namespace.

Perl has open namespaces . You can add functions or variables to a namespace at any point, either with a new package declaration:

... or by fully qualifying function names at the point of declaration:

You can add to a package at any point during compilation or runtime, regardless of the current file, though building up a package from multiple separate declarations can make code difficult to spelunk.

Namespaces can have as many levels as your organizational scheme requires, though namespaces are not hierarchical. The only relationship between packages is semantic, not technical. Many projects and businesses create their own top-level namespaces. This reduces the possibility of global conflicts and helps to organize code on disk. For example:

• StrangeMonkey is the project name
• StrangeMonkey::UI contains top-level user interface code
• StrangeMonkey::Persistence contains top-level data management code
• StrangeMonkey::Test contains top-level testing code for the project

... and so on.

Perl usually does what you expect, even if what you expect is subtle. Consider what happens when you pass values to functions:

Outside of the function, $name contains Chuck , even though the value passed into the function gets reversed into kcuhC . You probably expected that. The value of $name outside the function is separate from the $name inside the function. Modifying one has no effect on the other.

Consider the alternative. If you had to make copies of every value before anything could possibly change them out from under you, you'd have to write lots of extra defensive code.

Yet sometimes it's useful to modify values in place. If you want to pass a hash full of data to a function to modify it, creating and returning a new hash for each change could be troublesome (to say nothing of inefficient).

Perl 5 provides a mechanism by which to refer to a value without making a copy. Any changes made to that reference will update the value in place, such that all references to that value can reach the new value. A reference is a first-class scalar data type in Perl 5 which refers to another first-class data type.

Scalar References

The reference operator is the backslash ( \ ). In scalar context, it creates a single reference which refers to another value. In list context, it creates a list of references. To take a reference to $name :

You must dereference a reference to evaluate the value to which it refers. Dereferencing requires you to add an extra sigil for each level of dereferencing:

The double scalar sigil ( $$ ) dereferences a scalar reference.

While in @_ , parameters behave as aliases to caller variables Remember that for loops produce a similar aliasing behavior. , so you can modify them in place:

You usually don't want to modify values this way—callers rarely expect it, for example. Assigning parameters to lexicals within your functions removes this aliasing behavior.

Saving Memory with References

Modifying a value in place, or returning a reference to a scalar can save memory. Because Perl copies values on assignment, you could end up with multiple copies of a large string. Passing around references means that Perl will only copy the references—a far cheaper operation.

Complex references may require a curly-brace block to disambiguate portions of the expression. You may always use this syntax, though sometimes it clarifies and other times it obscures:

If you forget to dereference a scalar reference, Perl will likely coerce the reference. The string value will be of the form SCALAR(0x93339e8) , and the numeric value will be the 0x93339e8 portion. This value encodes the type of reference (in this case, SCALAR ) and the location in memory of the reference.

References Aren't Pointers

Perl does not offer native access to memory locations. The address of the reference is a value used as an identifier. Unlike pointers in a language such as C, you cannot modify the address or treat it as an address into memory. These addresses are only mostly unique because Perl may reuse storage locations as it reclaims unused memory.

Array References

Array references are useful in several circumstances:

• To pass and return arrays from functions without flattening
• To create multi-dimensional data structures
• To avoid unnecessary array copying
• To hold anonymous data structures

Use the reference operator to create a reference to a declared array:

Any modifications made through $cards_ref will modify @cards and vice versa. You may access the entire array as a whole with the @ sigil, whether to flatten the array into a list or count its elements:

Access individual elements by using the dereferencing arrow ( -> ):

The arrow is necessary to distinguish between a scalar named $cards_ref and an array named @cards_ref . Note the use of the scalar sigil ( Variable Sigils ) to access a single element.

Doubling Sigils

An alternate syntax prepends another scalar sigil to the array reference. It's shorter, if uglier, to write my $first_card = $$cards_ref[0] ; .

Use the curly-brace dereferencing syntax to slice ( Array Slices ) an array reference:

You may omit the curly braces, but their grouping often improves readability.

To create an anonymous array—without using a declared array—surround a list of values with square brackets:

This array reference behaves the same as named array references, except that the anonymous array brackets always create a new reference. Taking a reference to a named array always refers to the same array with regard to scoping. For example:

... both $sunday_ref and $monday_ref now contain a dessert, while:

... neither $sunday_ref nor $monday_ref contains a dessert. Within the square braces used to create the anonymous array, list context flattens the @meals array into a list unconnected to @meals .

Hash References

Use the reference operator on a named hash to create a hash reference :

Access the keys or values of the hash by prepending the reference with the hash sigil % :

Access individual values of the hash (to store, delete, check the existence of, or retrieve) by using the dereferencing arrow or double sigils:

Use the array sigil ( @ ) and disambiguation braces to slice a hash reference:

Create anonymous hashes in place with curly braces:

As with anonymous arrays, anonymous hashes create a new anonymous hash on every execution.

Watch Those Braces!

The common novice error of assigning an anonymous hash to a standard hash produces a warning about an odd number of elements in the hash. Use parentheses for a named hash and curly brackets for an anonymous hash.

Automatic Dereferencing

As of Perl 5.14, Perl can automatically dereference certain references on your behalf. Given an array reference in $arrayref , you can write:

Given an expression which returns an array reference, you can do the same:

The same goes for the array operators pop , shift , unshift , splice , keys , values , and each and the hash operators keys , values , and each .

If the reference provided is not of the proper type—if it does not dereference properly—Perl will throw an exception. While this may seem more dangerous than explicitly dereferencing references directly, it is in fact the same behavior:

Function References

Perl 5 supports first-class functions in that a function is a data type just as is an array or hash. This is most obvious with function references , and enables many advanced features ( Closures ). Create a function reference by using the reference operator on the name of a function:

Without the function sigil ( & ), you will take a reference to the function's return value or values.

Create anonymous functions with the bare sub keyword:

The use of the sub builtin without a name compiles the function as normal, but does not install it in the current namespace. The only way to access this function is via the reference returned from sub . Invoke the function reference with the dereferencing arrow:

Perl 4 Function Calls

An alternate invocation syntax for function references uses the function sigil ( & ) instead of the dereferencing arrow. Avoid this syntax; it has subtle implications for parsing and argument passing.

Think of the empty parentheses as denoting an invocation dereferencing operation in the same way that square brackets indicate an indexed lookup and curly brackets cause a hash lookup. Pass arguments to the function within the parentheses:

You may also use function references as methods with objects ( Moose ). This is useful when you've already looked up the method ( Reflection ):

Filehandle References

When you use open 's (and opendir 's) lexical filehandle form, you deal with filehandle references. Internally, these filehandles are IO::File objects. You can call methods on them directly. As of Perl 5.14, this is as simple as:

You must use IO::File; in 5.12 to enable this and use IO::Handle; in 5.10 and earlier. Even older code may take references to typeglobs:

This idiom predates lexical filehandles (introduced with Perl 5.6.0 in March 2000). You may still use the reference operator on typeglobs to take references to package-global filehandles such as STDIN , STDOUT , STDERR , or DATA —but these are all global names anyhow.

Prefer lexical filehandles when possible. With the benefit of explicit scoping, lexical filehandles allow you to manage the lifespan of filehandles as a feature of Perl 5's memory management.

Reference Counts

Perl 5 uses a memory management technique known as reference counting . Every Perl value has a counter attached. Perl increases this counter every time something takes a reference to the value, whether implicitly or explicitly. Perl decreases that counter every time a reference goes away. When the counter reaches zero, Perl can safely recycle that value.

How does Perl know when it can safely release the memory for a variable? How does Perl know when it's safe to close the file opened in this inner scope:

Within the inner block in the example, there's one $fh . (Multiple lines in the source code refer to it, but only one variable refers to it: $fh .) $fh is only in scope in the block. Its value never leaves the block. When execution reaches the end of the block, Perl recycles the variable $fh and decreases the reference count of the contained filehandle. The filehandle's reference count reaches zero, so Perl recycles it to reclaim memory, and calls close() implicitly.

You don't have to understand the details of how all of this works. You only need to understand that your actions in taking references and passing them around affect how Perl manages memory (see Circular References ).

References and Functions

When you use references as arguments to functions, document your intent carefully. Modifying the values of a reference from within a function may surprise the calling code, which doesn't expect anything else to modify its data. To modify the contents of a reference without affecting the reference itself, copy its values to a new variable:

This is only necessary in a few cases, but explicit cloning helps avoid nasty surprises for the calling code. If you use nested data structures or other complex references, consider the use of the core module Storable and its dclone ( deep cloning ) function.

Nested Data Structures

Perl's aggregate data types—arrays and hashes—allow you to store scalars indexed by integer or string keys. Perl 5's references ( References ) allow you to access aggregate data types through special scalars. Nested data structures in Perl, such as an array of arrays or a hash of hashes, are possible through the use of references.

Use the anonymous reference declaration syntax to declare a nested data structure:

Commas are Free

Perl allows but does not require the trailing comma so as to ease adding new elements to the list.

Use Perl's reference syntax to access elements in nested data structures. The sigil denotes the amount of data to retrieve, and the dereferencing arrow indicates that the value of one portion of the data structure is a reference:

The only way to nest a multi-level data structure is through references, so the arrow is superfluous. You may omit it for clarity, except for invoking function references:

Use disambiguation blocks to access components of nested data structures as if they were first-class arrays or hashes:

... or to slice a nested data structure:

Whitespace helps, but does not entirely eliminate the noise of this construct. Use temporary variables to clarify:

... or use for 's implicit aliasing to $_ to avoid the use of an intermediate reference:

perldoc perldsc , the data structures cookbook, gives copious examples of how to use Perl's various data structures.

Autovivification

When you attempt to write to a component of a nested data structure, Perl will create the path through the data structure to the destination as necessary:

After the second line of code, this array of arrays of arrays of arrays contains an array reference in an array reference in an array reference in an array reference. Each array reference contains one element. Similarly, treating an undefined value as if it were a hash reference in a nested data structure will make it so:

This useful behavior is autovivification . While it reduces the initialization code of nested data structures, it cannot distinguish between the honest intent to create missing elements in nested data structures and typos. The autovivification pragma ( Pragmas ) from the CPAN lets you disable autovivification in a lexical scope for specific types of operations.

You may wonder at the contradiction between taking advantage of autovivification while enabling strict ures. The question is one of balance. Is it more convenient to catch errors which change the behavior of your program at the expense of disabling error checks for a few well-encapsulated symbolic references? Is it more convenient to allow data structures to grow rather than specifying their size and allowed keys?

The answers depend on your project. During early development, allow yourself the freedom to experiment. While testing and deploying, consider an increase of strictness to prevent unwanted side effects. Thanks to the lexical scoping of the strict and autovivification pragmas, you can enable these behaviors where and as necessary.

You can verify your expectations before dereferencing each level of a complex data structure, but the resulting code is often lengthy and tedious. It's better to avoid deeply nested data structures by revising your data model to provide better encapsulation.

Debugging Nested Data Structures

The complexity of Perl 5's dereferencing syntax combined with the potential for confusion with multiple levels of references can make debugging nested data structures difficult. Two good visualization tools exist.

The core module Data::Dumper converts values of arbitrary complexity into strings of Perl 5 code:

This is useful for identifying what a data structure contains, what you should access, and what you accessed instead. Data::Dumper can dump objects as well as function references (if you set $Data::Dumper::Deparse to a true value).

While Data::Dumper is a core module and prints Perl 5 code, its output is verbose. Some developers prefer the use of the YAML::XS or JSON modules for debugging. They do not produce Perl 5 code, but their outputs can be much clearer to read and to understand.

Circular References

Perl 5's memory management system of reference counting ( Reference Counts ) has one drawback apparent to user code. Two references which eventually point to each other form a circular reference that Perl cannot destroy on its own. Consider a biological model, where each entity has two parents and zero or more children:

Both $alice and $robert contain an array reference which contains $cianne . Because $cianne is a hash reference which contains $alice and $robert , Perl can never decrease the reference count of any of these three people to zero. It doesn't recognize that these circular references exist, and it can't manage the lifespan of these entities.

Either break the reference count manually yourself (by clearing the children of $alice and $robert or the parents of $cianne ), or use weak references . A weak reference is a reference which does not increase the reference count of its referent. Weak references are available through the core module Scalar::Util . Its weaken() function prevents a reference count from increasing:

Now $cianne will retain references to $alice and $robert , but those references will not by themselves prevent Perl's garbage collector from destroying those data structures. Most data structures do not need weak references, but when they're necessary, they're invaluable.

Alternatives to Nested Data Structures

While Perl is content to process data structures nested as deeply as you can imagine, the human cost of understanding these data structures and their relationships—to say nothing of the complex syntax—is high. Beyond two or three levels of nesting, consider whether modeling various components of your system with classes and objects ( Moose ) will allow for clearer code.

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In most of the programming language True and False are considered as the boolean values. But Perl does not provide the type boolean for True and False. In general, a programmer can use the term “boolean” when a function returns either True or False. Like conditional statements(if, while, etc.) will return either true or false for the scalar values.

Output:  

True Values: Any non-zero number i.e. except zero are True values in the Perl language. String constants like ‘true’ , ‘false’ , ‘ ‘ (string having space as the character), ’00’ (2 or more 0 characters) and “0\n” (a zero followed by a newline character in string) etc. also consider true values in Perl.

• Example:  

False Values: Empty string or s tring contains single digit 0 or undef value and zero are considered as the false values in perl.

Note: For the conditional check where the user has to compare two different variables, if they are not equal it returns False otherwise True .

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Operators provide a simple syntax for manipulating values. Many of the Perl 6 operators will be familiar, especially to Perl programmers.

Assignment and Binding

The = operator is for ordinary assignment. It creates a copy of the values on the right-hand side and assigns them to the variables or data structures on the left-hand side:

$copy and $original both have the same value, and @copies has a copy of every element in @originals .

The := operator is for binding assignment. Instead of copying the value from one variable or structure to the other, it creates an alias. An alias is an additional entry in the symbol table with a different name for the one container:

In this example, any change to $a also changes $b , because they’re just two separate names for the same container. Binding assignment requires the same number of elements on both sides, so both of these would be an error:

The ::= operator is a variant of the binding operator that binds at compile time.

Arithmetic Operators

The binary arithmetic operators are addition ( + ), subtraction ( - ), multiplication ( * ), division ( / ), modulus ( % ), and exponentiation ( ** ). Each has a corresponding assignment operator ( += , -= , *= , /= , %= , **= ) that combines the arithmetic operation with assignment:

The unary arithmetic operators are the prefix and postfix autoincrement ( ++ ) and autodecrement ( -- ) operators. The prefix operators modify their argument before it’s evaluated, and the postfix operators modify it afterward:

String Operators

The ~ operator concatenates strings. The corresponding ~= operator concatenates the right-hand side of the assignment to the end of the string:

The x operator replicates strings. It always returns a string no matter whether the left side of the operation is a single element or a list. The following example assigns the string “LintillaLintillaLintilla”:

The corresponding x= operator replicates the original string and assigns it back to the original variable:

The xx operator replicates lists. It returns a list no matter whether it operates on a list of elements or a single element. The following example assigns a list of three elements to @array , each with the value “Lintilla”:

The corresponding xx= operator creates a list that contains the specified number of copies of every element in the original array and assigns it back to the array variable:

Each comparison operator has two forms, one for numeric comparisons and one for string comparisons. The comparison operators are greater-than ( > , gt ), less-than ( < , lt ), greater-than-or-equal ( >= , ge ), less-than-or-equal ( <= , le ), equality ( = = , eq ), and inequality ( != , ne ). Each returns a true value if the relation is true and a false value otherwise. The generic comparison operator ( <=> , cmp ) returns 0 if the two arguments are equal, 1 if the first is greater, and -1 if the second is greater.

Logical Operators

The binary logical operators test two values and return one value or the other depending on certain truth conditions. They’re also known as the short-circuit operators because the right-hand side will never be evaluated if the overall truth value can be determined from the left-hand side. This makes them useful for conditionally assigning values or executing code.

The AND relation has the && operator and the low-precedence and operator. If the left-hand side is false, its value is returned. If the left-hand value is true, the right-hand side is evaluated and its value is returned:

The OR relation has the || operator and the low-precedence or operator. The left-hand value is returned if it is true, otherwise the right-hand value is evaluated and returned:

A variant of the OR relation tests for definedness instead of truth. It uses the // operator and the low-precedence err operator. The left-hand value is returned if it is defined, otherwise the right-hand side is evaluated and its value returned:

The XOR relation has the ^^ operator and the low-precedence xor operator. It returns the value of the true operand if any one operand is true and a false value if both are true or neither is true. xor isn’t short-circuiting like the others, because it always has to evaluate both arguments to know if the relation is true:

Perl 6 also has boolean variants of the logical operators: ?& (AND), ?| (OR), and ?^ (XOR). These always return a true or false value.

Context Forcing Operators

The context of an expression specifies the type of value it is expected to produce. An array expects to be assigned multiple values at the same time, so assignment to an array happens in “list” context. A scalar variable expects to be assigned a single value, so assignment to a scalar happens in “scalar” context. Perl expressions often adapt to their context, producing values that fit with what’s expected.

Contexts have proven to be valuable tools in Perl 5, so Perl 6 has a few more added. Void context still exists. Scalar context is subdivided into boolean, integer, numeric, string, and object contexts. Aside from flattening list context and hashlist context, which we mentioned earlier, list context is further subdivided into non-flattening list context and lazy list context.

Expects no value.

Expects a single value. Composite values are automatically referenced in scalar context.

Expects a true or false value. This includes the traditional definitions of truth—where 0 , undef , and the empty string are false and all other values are true—and values flagged with the properties true or false .

Expects a number, whether it’s an integer or floating-point, and whether it’s decimal, binary, octal, hex, or some other base.

Expects an integer value. Strings are treated as numeric and floating- point numbers are truncated.

Expects a string value. It interprets any information passed to it as a string of characters.

Expects an object, or more specifically, a reference to an object.

Expects a collection of values. Any single value in list context is treated as a one-element list.

Expects a list of objects. It treats arrays, hashes, and other composite values as discrete entities.

Expects a list, but flattens out arrays and hashes into their component parts.

Expects a list of pairs. A simple list in hashlist context pairs up alternating elements.

Expects a list, just like non-flattening list context, but doesn’t require all the elements at once.

The unary context operators force a particular context when it wouldn’t otherwise be imposed. Most of the time the default context is the right one, but at times you might want a little more control.

The unary ? operator and the low-precedence true force boolean context. Assignment of a scalar to a scalar only imposes generic scalar context, so the value of $number is simply copied. With the ? operator, you can force boolean context and assign the truth value of the variable instead of the numeric value:

The unary ! operator and the low-precedence not also force boolean context, but they negate the value at the same time. They’re often used in a boolean context, where only the negating effect is visible:

The unary + operator forces numeric context, and - forces negative numeric context:

The unary ~ operator forces string context:

Bitwise Operators

Perl 6 has two sets of bitwise operators, one for integers and one for strings. The integer bitwise operators are +& , +| , and +^ . Notice the combination of the AND, OR, and XOR relation symbols with the general numeric symbol + (the unary numeric context operator). There are also the numeric bitwise shift operators << and >> .

The string bitwise operators are ~& , ~| , and ~^ . These combine the AND, OR, and XOR relation symbols with the general string symbol ~ (the same symbol as string concatenation and the unary string context operator).

Each of the bitwise operators has an assignment counterpart +&= , +|= , +^= , <<= , >>= , ~&= , ~|= , and ~^= .

Conditional

The ternary ??: : operator evaluates either its second or third operand, depending on whether the first operand evaluates as true or false. It’s basically an if-then-else statement acting as an expression:

Vector Operators

The vector operators are designed to work with lists. They’re simply modified versions of the standard scalar operators. Every operator has a vectorized version, even user-defined operators. They have the same basic forms as their scalar counterparts, but are marked with the bracketing characters » and «, [ 8 ] or their plain-text equivalents >> and << .

So, the vectorized addition operator is >>+<< . Vector operators impose list context on their operands and distribute their operations across all the operands’ elements. Vector addition takes each element from the first list and adds it to the corresponding element in the second list:

The resulting array contains the sums of each pair of elements, as if each pair were added with the scalar operator:

If one side of a vector operation is a simple scalar, it is distributed across the list as if it were a list of identical elements:

At the simplest level, junction operators are no more than AND, OR, XOR, and NOT for values instead of expressions. The binary junction operators are & (AND), | (OR), and ^ (XOR). [ 9 ]

So while || is a logical operation on two expressions:

| is the same logical relation between two values:

In fact, those two examples have exactly the same result: they return true when $value is 1 or 2 and false otherwise. In the common case that’s all you’ll ever need to know.

But junctions are a good deal more powerful than that, once you learn their secrets. In scalar context, a junctive operation doesn’t return an ordinary single value, it returns a composite value containing all of its operands. This return value is a junction, and it can be used anywhere a junction operation is used:

Here, the variable $junc is used in place of 1 | 2 , and has exactly the same effect as the earlier example.

A junction is basically just an unordered set with a logical relation defined between its elements. Any operation on the junction is an operation on the entire set. Table 4-1 shows the way the four different types of junctions interact with other operators.

Table 4-1. Junctions

Function	Operator	Relation	Meaning
		AND	Operation must be true for all values.
		OR	Operation must be true for at least one value.
		XOR	Operation must be true for exactly one value.
		NOT	Operation must be false for all values.

The simplest possible example is the result of evaluating a junction in boolean context. The operation on the set is just “is it true?” This operation on an all junction is true if all the values are true:

So, if $a and $b are both true, the result is true.

On an any junction, it’s true if any one value is true:

So, if $a or $b is true or if both are true, the result is true.

On a one junction, it’s true only if exactly one value is true:

So, if either $a or $b is true, the result is true. But, if $a and $b are both true or neither is true, the result is false.

On a none junction, it’s true only when none of the values are true—that is, when all the values are false.

So, if $a and $b are both false, the result is true.

Ordinary arithmetic operators interact with junctions much like vector operators on arrays. A junction distributes the operation across all of its elements:

Junctions can be combined to produce compact and powerful logical comparisons. If you want to test that two sets have no intersection, you might do something like:

which tests that all of the elements of the first set are equal to none of the elements of the second set. Translated to ordinary logical operators that’s:

If you want to get back a flat list of values from a junction, use the .values method:

The .dump method returns a string that shows the structure of a junction:

The .pick method selects one value from an any junction or a one junction that has exactly one value, and returns it as an ordinary scalar:

On an all junction, a none junction, or a one junction with more than one value, .pick returns undef . [ 10 ]

Smart Match

The binary ~~ operator makes a smart match between its two terms. It returns a true value if the match is successful and a false value if the match fails. [ 11 ] The negated smart match operator !~ does the exact opposite: it returns true if the match fails and false if it is successful. The kind of match a smart match does is determined by the kind of arguments it matches. If the types of the two arguments can’t be determined at compile time, the kind of match is determined at runtime. In all but two cases smart match is a symmetric operator, so you can reverse A ~~ B to B ~~ A and it will have the same truth value.

Matching scalars

Any scalar value or any code that results in a scalar value matched against a string tests for string equality. The following match is true if $string has the value “Ford”:

Any scalar value matched against a numeric value tests for numeric equality. The following is true if $number has the numeric value 42, or the string value “42”:

An expression that results in the value 42 is also true:

Any scalar value matched against an undefined value checks for definedness. The following matches are true if $value is an undefined value and false if $value is any defined value:

Any scalar value matched against a rule (regex) does a pattern match. The following match is true if the sequence “towel” can be found anywhere within $string :

Any scalar value matched against a substitution does that substitution on the value. This means the value has to be modifiable. The following match is true if the substitution succeeds on $string and false if it fails:

Any scalar value matched against a boolean value simply takes the truth value of the boolean. The following match will always be true, because the boolean on the right is always true: [ 12 ]

The boolean value on the right must be an actual boolean: the result of a boolean comparison or operation, the return value of a not or true function, or a value forced into boolean context by ! or ? . The boolean value also must be on the right; a boolean on the left is treated as an ordinary scalar value.

Matching lists

Any scalar value matched against a list compares each element in sequence. The match is true if at least one the element of the list would match in a simple expression-to-expression match. The following match is true if $value is the same as any of the three strings on the right:

This match is short-circuiting. It stops after the first successful match. It has the same truth value as a series of or -ed matches:

A list can contain any combination of elements: scalar values, rules, boolean expressions, arrays, hashes, etc.:

A match of a list against another list sequentially compares each element in the first list to the corresponding element in the second list. The match is true if every element of the first list matches the corresponding element in the second list. The following match is true, because the two lists are identical:

The two lists don’t have to be identical, as long as they’re the same length and their corresponding elements match:

The list-to-list match is also short-circuiting. It stops after the first failed match. This has the same truth value as a series of single-element smart matches linked by and :

Matching arrays

A nonnumeric expression matched against an array sequentially searches for that value in the array. The match is true if the value is found. If @array contains the values “Zaphod”, “Ford”, and “Trillian”, the following match is true when $value is the same as any of those three strings:

An integer value matched against an array tests the truth of the value at that numeric index. The following match is true if the element @array[2] exists and has a true value:

An integer value matched against an array reference also does an index lookup:

This match is true, because the third element of the array reference is a true value.

An array matches just like a list of scalar values if it’s flattened with the * operator. [ 13 ]

So, the following example searches the array for an element with the value 2 , instead of doing a index lookup:

An array matched against a rule does a pattern match across the array. The match is true if any element of the array matches the rule. If “Trillian”, “Gillian”, or “million” is an element of @array , the following match is true, no matter what the other elements are:

A match of an array against an array sequentially compares each element in the first array to the corresponding element in the second array:

This match is true if the two arrays are the same length and @humans[0] matches @vogons[0] , @humans[1] matches @vogons[1] , etc.

Matching hashes

A hash matched against any scalar value tests the truth value of the hash entry with that key:

This match is true if the element %hash{$key} exists and has a true value.

A hash matched against a rule does a pattern match on the hash keys:

This match is true if at least one key in %hash matches “bl”.

A hash matched against a hash checks for intersection between the keys of the two hashes:

So, this match is true if at least one key from %vogons is also a key of %humans . If you want to see that two hashes have exactly the same keys, match their lists of keys:

A hash matched against an array checks a slice of a hash to see if its values are true. The match is true if any element of the array is a key in the hash and the hash value for that key is true:

If @array has one element `blue’ and %hash has a corresponding key `blue', the match is true if %hash{'blue'} has a true value, but false if %hash{'blue'} has a false value (`0', an empty string or undef).

Matching junctions

An expression matched against an any junction is a recursive disjunction. The match is true if at least one of the elements of the list would match in a simple expression-to-expression match:

This example matches if $value is the same as any of the three strings on the right. The effect of this comparison is the same as a simple comparison to a list, except that it isn’t guaranteed to compare in any particular order.

A smart match of an all junction is only true when the expression matches every value in the junction:

A smart match of a one junction is only true when the expression matches exactly one value in the junction:

A smart match of a none junction is true when it doesn’t match any values in the junction:

An any junction matched against another any junction is a recursive disjunction of every value in the first junction to every value in the second junction. The match is true if at least one value of the first junction matches at least one value in the second junction:

This match is true, because “Trillian” is in both junctions.

Matching objects

An object matched against a class name is true if the object belongs to that class, or inherits from that class. It’s essentially the same as calling the .isa method on the object:

An object calls a method it’s matched against. The match is true if the method returns a true value:

Matching subroutines

Any expression matched against a subroutine tests the return value of the subroutine. If the subroutine takes no arguments it is treated as a simple boolean:

If the subroutine has a one argument signature and it is the same variable type as the expression, the subroutine is called with the expression as its argument. The return value of the subroutine determines the truth of the match:

Referencing (or Not)

The unary \ operator returns a reference to its operand. The referencing operator isn’t needed very often, since scalar context automatically generates references to arrays, hashes, and functions, but it is still needed in flattening contexts and other contexts that don’t auto-reference:

The unary * operator (that’s the “splat” operator) flattens a list in a context where it would usually be taken as a reference. On an rvalue, * causes the array to be treated as a simple list:

Since the @combo array contains an arrayref and a hashref, an ordinary binding assignment of @combo to @a treats @combo as a single element and binds it to @a . With the flattening operator, the @combo array is treated as a simple list, so each of its elements are bound to a separate element on the left-hand side. @b is bound to the original @array and %c is bound to the original %hash .

On an lvalue, * tells the array to “slurp” all available arguments. An ordinary binding of two arrays to two arrays simply binds the first element on the right-hand side to the first element on the left-hand side, and the second to the second:

So, @a is bound to @c , and @b is bound to @d . With the * operator, the first element on the left-hand side sucks up all the elements on the right-hand side, so @a contains all the elements from @c and @d .

One common use for * is in defining subroutine and method signatures, as you will see in Section 4.4 later in this chapter.

Zip Operator

The ¦ operator takes two lists (arrays, hash keys, etc.) and returns a single list with alternating elements from each of the original lists. This allows loops and other iterative structures to iterate through the elements of several lists at the same time:

There is no equivalent ASCII operator for the zip operator, but the zip function is much more fully featured than the operator. It is described in Section 4.3.2.3 later in this chapter.

[ 8 ] These are the Unicode RIGHT POINTING GUILLEMET (U+00BB) and LEFT POINTING GUILLEMET (U+00AB) characters.

[ 9 ] There isn’t an operator for junctive NOT, but there is a function, as you’ll see shortly.

[ 10 ] With some levels of error strictness, it may raise an exception.

[ 11 ] This is an oversimplification. Some matches return a more complex value, but in boolean context it will always evaluate as true for a successful match, and false for a failed match.

[ 12 ] At the moment this relation won’t seem particularly useful. It makes much more sense when you realize that the switch statement duplicates all the smart match relations. More on that in Section 4.3.1.3 later in this chapter.

[ 13 ] See Section 4.2.12 later in the chapter.

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An operator is the element affecting operands in a Perl expression and causes Perl to execute an operation on one of more operands. In the expression $a + 5 , $a and 5 are the operands and + is the operation causing the addition operation.

Perl programming can be accomplished by directly executing Perl commands at the shell prompt or by storing them in a text file with the .pl extension, and then executing it as a Perl script via perl file.pl .

Perl supports many operator types. Following is a list of frequently used operators.

Arithmetic Operators

Manipulate numeric scalar values.

• + Addition - Adds the operand values from either side of the operator
• - Subtraction - Subtracts the right operand from the left operand
• - Negation - When taking a single operand (unary), it calculates the negative value
• * Multiplication - Multiplies the operand values from either side of the operator
• / Division - Divides the left operand by the right operand
• % Modulus - Divides the left operand by the right operand and returns remainder
• ** Exponent - Calculates the left operand to the power of the right operand

Comparison Operators

Used to compare two scalar string or scalar numeric values. Comparison or relational operators are discussed in the Conditional Decisions section of this tutorial. Please review them in that section.

Assignment Operators

Used to assign scalar or array data to a data structure.

• = Simple assignment - assigns values from right side operands and operators to left side operand
• += Addition and assign - add right operand to left operand and assign to left operand
• -= Substract and assign - substract right operand from left operand and assign to left operand
• *= Multiply and assign - multiply right operand by left operand and assign to left operand
• /= Divide and assign - divide left operand by right operand and assign to left operand
• %= Modulus and assign - divide left operand by right operand and assign remainder to left operand
• **= Exponent and assign - calculates the left operand to the power of the right operand and assign to left operand
• ++ Autoincrement - increases unary operand value by one. E.g. $x++ or ++$x gives 11
• -- Autodecrement - decreases unary operand value by one. E.g. $x-- or --$x gives 9

Note that $x++ is considered post-autoincrement , and ++$x is considered pre-autoincrement :

• $a = $x++ will assign $a with 10 and $x with 11 (autoincrement $x after assigning value to $a )
• $a = ++$x will assign $x with 11 then assign $a with 11 (autoincrement $x before assigning value to $a )

Similarly with -- (autodecrement):

• $a = $x-- will assign $a with 10 and $x with 0 (autodecrement $x after assigning value to $a )
• $a = --$x will assign $x with 9 then assign $a with 9 (autodecrement $x before assigning value to $a )

Bitwise Operators

Manipulate numeric scalar values at the bit level.

Bitwise operators treat numeric operands as binary numbers and perform bit by bit operations. Scalars can be assigned with decimal, binary (with prefix 0b ) or hexadecimal (with prefix 0x ).

• & AND - bitwise AND of the operand values from either side of the operator e.g. $b & $mask gives 0b1001
• | OR - bitwise OR of the operand values from either side of the operator e.g. $b | $mask gives 0b1111
• ^ XOR - bitwise XOR of the operand values from either side of the operator e.g. $b & $mask gives 0b1001
• ~ NOT - bitwise INVERT (unary operator) inverts each bit of the left operand e.g. ~$b give 0b10100110
• << SHIFT LEFT - bitwise SHIFT LEFT the left operand, right operand times e.g. $b << 1 give 0b101001100
• >> SHIFT RIGHT - bitwise SHIFT RIGHT the left operand, right operand times e.g. $b >> 1 give 0b01010011

Logical Operators

Evaluate logical relations between operands.

Logical operators calculate a logical value - TRUE or FALSE, per the values of their operands.

• and Logical AND operator - return TRUE if both the operands are true, otherwise FALSE
• && Logical AND operator - return TRUE if both the operands are true, otherwise FALSE
• or Logical OR operator - return TRUE if either one of the operands is true, otherwise FALSE
• || Logical OR operator - return TRUE if either one of the operands is true, otherwise FALSE
• xor Logical XOR operator - return TRUE if one of the operands is true and the other is false, otherwise FALSE
• not Logical NOT operator (unary operator)- return TRUE if the operand is false, otherwise FALSE
• ! Logical NOT operator (unary operator)- return TRUE if the operand is false, otherwise FALSE

String Operators

Manipulate string scalar values.

• . String concatenation operator - concatenate the left and right operands e.g. $a . $b gives "world hello"
• x String repetition operator - return the left operand repeated the number of times specified by the right operator. E.g. $b x 3 gives "hellohellohello"

Miscellaneous Operators

• .. The range operator - returns an array of values reflecting the sequential range beteen the two operands. For numeric operands, the values are incremented by 1 from the left operand to the right operand. For letters (lowercase or uppercase), the values are incremented in alphabetical order.

Follow these instructions and print the result after each step.

• Assign scalar $a to a starting value of 5. Print value of $a .
• Add 6 to the previous result. Print the new result.
• Multiply the previous result by 2. Print the new result.
• Autoincrement the previous result. Print the new result.
• Substract 9 from the previous result. Print the new result.
• Divide the previous result by 7. Print the new result.

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Constants and read-only variables in Perl

Readonly::xs, treat upper-case variables as constants, the readonly module, the constant pragma, other ways to create constants.

Published on 2013-09-14

Author: Gabor Szabo

"be consistent"

by (Archbishop)

) NODE.title = Truth and Falsehood NODE.owner = 830549 N.title = monktitlebar sitedoclet N.owner = 17342 -->

( = : , )

In Perl, the following values are in boolean context: are . Boolean context is a scalar context. Therefore the following things, when evaluated in boolean/scalar context, are also : , which evaluates to . A true value negated by or , plus many of Perl's builtins/operators, return a : When evaluated as a string it is treated as , but as a number, it is treated as 0, without causing any warnings. the same as : even though is also as a string and 0 as a number, Perl's special false value is a value, and does produce when you try to use it as a string or number. also causes warnings when you try to use it as a number.) When the Perl documentation says that a operator or function returns "a false value" or "a true value" (or more simply, "false" or "true"), it may return any of the above values. Boolean context is provided by, for example, ) { ... }, ? ... : ..., or if ;. If you are unsure about Perl's concept of context, see e.g. the , the section "Context" in Chapter 2 of , or the section in . To test whether a value is or not, use . Note that "false" values like the empty string or the number 0 are still defined. Perl's special false value is as well. To test whether a hash contains a certain key or not, use . Note that a key can exist in a hash, but the value stored for that key can be . If you attempt to access a hash key that does not exist, Perl will also return , which is why the distinction between and is important, especially if you're going to be iterating over the of the hash. To remove a key from a hash, use . To get the length of a string, use . For example, the test allows you to differentiate between and , which would otherwise both be false. Note: As of Perl 5.12, if you give the function an value, it will return without producing a warning, which is helpful in boolean tests. (In Perl versions before 5.12, would produce a warning when given .) To get the size of an array, use , as in , or use the array in a scalar context, for example . To test whether an array is empty or not, simply use it in a boolean context, such as . (Note: returns the index of the last element of the array, so it is not the same - see for details.) If a hash is empty, using the hash in scalar/boolean context (e.g. ) will return a false value, regardless of the Perl version. To portably get the number of keys in a hash, use in a scalar context, as in or . As of Perl 5.26, it is also possible to say directly to get the number of keys (previous versions of Perl would return some internal information about the hash: the number of used buckets and the number of allocated buckets). In a few special cases, Perl functions may wish to return the number zero, but want to do so in a way that this value is still "true" when tested in a boolean context. For example, this could be to indicate that an operation was successful, but its return value is zero. Two common ways to do this are for the function to return the strings or , which are both "true" under the above rules, but when used as a number, are zero. (The string is special-cased in Perl to not produce warnings about it not looking like a number.) To force some value into Perl's boolean values, you can use double negation, , also known as "bang bang" ( ). If you're unsure about precedence, use parentheses: . Normally, would be true, since in the normal case a reference is a true value. However, an object may choose to change its behavior for different operations, such as boolean context. So, in the presence of ing, may be false. Authors of classes are generally advised to use this feature in a way that makes the code DWIM (Do What I Mean). (You may disable overloading with , although in that form it will disable overloading.) Perl has a special feature where a variable may contain two different values, one string and one number, that are different. In this case, the string part of the variable is tested for truth. For example, if you use 's to create a variable with a numeric part of 0 (false) and a string part of (true), it will test true, and a variable with a numeric part 1 and a string part is false. Note that this feature is rarely made use of intentionally, so you usually don't have to worry about it. To inspect a variable for this property, use 's , and see 's output for all the gory details. for this suggestion!) Minor clarifications. Added section on overloading, thanks ! Updates according to 's reply, thanks! Added section on forcing boolean context. A few tiny tweaks here and there, updated and added a few more links. Added a bit more to the section "Context" and a bit more about . in , but was into a much longer text in (and is now harder to link to explicitly, IMHO). Truth and Falsehood or Code

Replies are listed 'Best First'.
by (Archbishop) on Aug 17, 2019 at 10:09 UTC
I've tested empty hashes on several perls here and they, like empty arrays, always appear to evaluate to zero as well. Is there a reason why you have expressed them differently here?
by (Archbishop) on Aug 17, 2019 at 10:12 UTC
Just because that's what the Perl documentation says. :
by (Archbishop) on Aug 17, 2019 at 10:19 UTC
is: which suggests that there may be some versions out there where the false value returned is not zero (or that there may be in future, intriguingly). I'll have to try to remember only to test the truth of empty hashes rather than their numerical value. :-)
by (Archbishop) on Aug 17, 2019 at 10:30 UTC
by (Parson) on Aug 17, 2019 at 22:38 UTC
As currently written, this neglects . I think it would be better to write the first paragraph in "Hash and Array Size" as: , as in , or use the array in a scalar context, for example . To test whether an array is empty or not, simply use it in a boolean context, such as . To get the largest index in an array, use . To explicitly resize an array, assign a value to . Perl normally grows arrays as needed, but explicitly assigning to can also shrink an array, discarding elements from the high end. I have tried to follow your style in the above text, but I expect that you will further edit that before using it, if you use it. The and tests also apply to array indexes, in the same way that they apply to hash keys. This can be explained briefly and would go well with adding mention of . And one minor stylistic quibble: I think of exceptions as "thrown" with because they can be "caught" with , but can warnings be similarly "caught" in Perl or would it be better to say that returns without a warning?
by (Archbishop) on Aug 18, 2019 at 06:46 UTC
The intent was a little different, as I mentioned at the very bottom of the node: There a section "Truth and Falsehood" in that was nice and short, and easy to link to whenever someone's question touched on Perl's concept of true/false, but that section was into a much longer block of text in , which IMHO makes it harder to provide a link to people who just want to know about that one topic. (The title is such that it's easy to link to as .) So I want to keep the main point of the node focused on just true/false, hence I tried to separate out the "Related Topics" so that the top is just a "TL;DR". . Yes, that's a good point, I'll add a brief mention of it, thanks. IMHO, the more detailed description you wrote starts getting a little off-topic to the main point of the node, so I'll keep it a little shorter. and tests also apply to array indexes Not entirely; from : Calling on array values is strongly discouraged. The notion of deleting or checking the existence of Perl array elements is not conceptually coherent, and can lead to surprising behavior. returns without producing Yes, good point, thanks!
by (Parson) on Aug 18, 2019 at 07:10 UTC
Fair enough, and the link to the reference manual makes up for trimming it. on arrays] The tests apply, even though the results can be surprising, mostly due to the fact that Perl's arrays have a contiguous range of index values. I suspect that is equivalent to . If not, then that is surprising and probably better left "under the rug". :-)
by on Nov 01, 2019 at 23:53 UTC
/s;say'
by (Canon) on Dec 17, 2019 at 22:13 UTC
by (Patriarch) on Aug 18, 2019 at 06:29 UTC
When's the last time you did ? Why would you ever do that? I suppose one could use , but that's a very bad practice. (A sub that's expected to return a scalar should do so even in list context.) So I'm rather baffled as to why this was mentioned. There are so many operators that can return false that would merit a mention before , such as scalar assignment, list assignment, , , and .
by (Archbishop) on Aug 18, 2019 at 07:04 UTC
, but that's a very bad practice. (A sub that's expected to return a scalar should do so even in list context.) I agree that s that are expected to return a scalar should do because of the problems with , but there are also some against in other cases - as , there are exceptions. I mostly mentioned it because the mentioned it, although with an attempt to improve the wording, as yourself in the past. I missed your ninja edits, but I hope I've cleared up your bafflement nonetheless. Please feel free to add any information you think is missing!
by (Parson) on Aug 18, 2019 at 07:13 UTC
, , , and all return false by returning an empty list, at least in a simple description.
by (Patriarch) on Aug 30, 2019 at 16:31 UTC
returns the count of matching elements. returns the count of elements it would return.
by (Deacon) on Nov 01, 2019 at 22:04 UTC
(Which gives a warning.) Now "$false" is exactly the same as "not 1". Or more explicit:
by (Archbishop) on Nov 01, 2019 at 23:02 UTC

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Perl - If else

Simple perl if conditional statement.

if the conditional statement is used to execute code block based on conditional expression statements.

condition_expression is always evaluated as true or false . if it is true , It executes a code block.

Here is an example

conditional_expression is a Boolean expression that returns true or false .

Perl if else conditional statements

if else in Perl is used to execute code statements based on true and false conditional expressions.

condition is a Boolean expression that returns true or false

Perl if else if statement example

Perl if condition with and operator, perl if condition string example, perl one line if statement.