perl multiple variable assignment

Home » Perl Variables

Perl Variables

Summary :  in this tutorial, you’ll learn about Perl variables , variable scopes, and variable interpolation.

To manipulate data in your program, you use variables.

Perl provides three types of variables: scalars, lists, and hashes to help you manipulate the corresponding data types including scalars, lists, and hashes .

You’ll focus on the scalar variable in this tutorial.

Naming variables

You use scalar variables to manipulate scalar data such as numbers and strings .

A scalar variable starts with a dollar sign ( $ ), followed by a letter or underscore, after that, any combination of numbers, letters, and underscores. The name of a variable can be up to 255 characters.

Perl is case-sensitive. The $variable and $Variable are different variables.

Perl uses the dollar sign ( $ ) as a prefix for the scalar variables because of the $  looks like the character S in the scalar. You use this tip to remember when you want to declare a scalar variable.

The following example illustrates valid variables:

However, the following variables are invalid in Perl.

Declaring variables

Perl doesn’t require you to declare a variable before using it.

For example, you can introduce a variable in your program and use it right away as follows:

In some cases, using a variable without declaring it explicitly may lead to problems. Let’s take a look at the following example:

The expected output was Your favorite color is red .  

However, in this case, you got Your favorite color is , because the $color and $colour are different variables. The mistake was made because of the different variable names.

To prevent such cases, Perl provides a pragma called strict that requires you to declare variable explicitly before using it. 

In this case, if you use the  my keyword to declare a variable and try to run the script, Perl will issue an error message indicating that a compilation error occurred due to the  $colour variable must be declared explicitly.

A variable declared with the  my keyword is a lexically scoped variable.

It means the variable is only accessible inside the enclosing block or all blocks nested inside the enclosing block. In other words, the variable is local to the enclosing block.

Now, you’ll learn a very important concept in programming called variable scopes.

Perl variable scopes

Let’s take a look at the following example:

In the example above:

• First, declared a global variable named  $color .
• Then, displayed the favorite color by referring to the $color variable. As expected, we get the red color in this case.
• Next, created a new block and declared a variable with the same name $color using the my keyword. The  $color variable is lexical. It is a local variable and only visible inside the enclosing block.
• After that, inside the block, we displayed the favorite color and we got the blue color. The local variable takes priority in this case.
• Finally, following the block, we referred to the $color variable and Perl referred to the  $color global variable.

If you want to declare global variables that are visible throughout your program or from external packages, you can use our   keyword as shown in the following code:

Perl variable interpolation

Perl interpolates variables in double-quoted strings. It means if you place a variable inside a double-quoted string, you’ll get the value of the variable instead of its name.

Perl interpolates the value of $amount into the string which is 20.

Note that Perl only interpolates scalar variables and arrays , not hashes . In addition, the interpolation is only applied to the double-quoted string, but not the single-quoted string.

In this tutorial, you have learned about Perl variables including naming and declaring scalar variables. You’ve also learned about variable scopes and variable interpolation.

Perl subroutine values - How to return multiple values from a Perl function

Perl subroutine FAQ: How do I return multiple values from a Perl subroutine (Perl function)?

One of the things I really like about Perl is that you can return multiple values from a function (and you don't have to create some type of artificial class to encapsulate them). Here's the basic way to return multiple values from a function/subroutine named foo :

As you can see from the assignment statement above, you can return these multiple values into three separate variables, $a , $b , $c , which is also very cool.

If you save this Perl code to a file and then run it you'll get this output:

As you can see from the output, the three values I return from the function are assigned to my three variables when I call the function.

Returning multiple values to an array

You can also assign an array to hold the multiple return values from a Perl function. You do that like this:

As you can see, most of the code is the same, except I now assign an array ( @arr ) to contain the three return values from my function.

Save this Perl code to a file and run it and you'll see this output:

As you can see from these examples, you can assign multiple return values from a Perl function either to separate variables, or a single array.

Help keep this website running!

• Perl subroutine - how to return multiple values
• How to access arguments to a Perl subroutine or function
• Perl - How to compare a string against multiple patterns
• Perl hash sort - How to sort a Perl hash by the hash value
• How to add an item to a Perl hash

books by alvin

• A gratitude and compassion meditation exercise, and living in the real world (Earth School)
• ZIO.fromOption returns Option[Nothing]; what is that? (Unit? None?)
• Lucid dream: A distraught woman who only spoke Spanish
• Scala: What is the Nothing data type?
• Maine Cabin Masters Black Pirate Flag (skull and crossbones)

Variables and Types

Basic syntax:.

The hash ( # ) sign starts a comment. Lines starting with = are interpreted as the start of a section of embedded documentation (pod - plain old text), and all subsequent lines until the next =cut are ignored by the compiler.

The semicolon ( ; ) is required to end each Perl statement.

The simplest way to print text to the screen is using the print command. Note that the parenthesis are notr required, so the next statements will produce the same result:

Note that print needs the newline character ( \n ) to be explicitly defined.

The double-quotes ( " ) are used to encapsulate a string to be printed, and it allows replacement of variables and special characters (like \n ) inside the string. This is called interpolation . The single-quote ( ' ) prevents variable value replacement.

Perl is a case sensitive programming language. Thus $World and $world are two different variables in Perl.

A Perl variable name starts with either $ , @ or % followed by zero or more letters, underscores, and digits.

Perl supports 3 kinds of variables:

• Scalars contain a single string or numeric value. The variable name must start with a $ .
• Arrays contain a randomly accessed ordered set of values. The variable name must start with a @ .
• Hashes contain (key, value) pairs efficiently accessed per key. The variable name must start with a % .

Note that there is no such thing as a boolean data type in Perl (more on this in later).

Assigning values to variable is done with the equal sign ( = ). The operand to the left of the = sign is the name of the variable, and the operand to the right is the value stored in the variable.

For example, some assignments to scalars:

Now, some assignments to array variables. To refer to a single element of an array, the variable name must start with a $ followed by the index of the element in square brackets ( [] ). The index of the first array element is 0.

Now, some assignments to hash variables. To refer to a single element of a hash, the variable name must start with a $ followed by the key of the requested element in curly brackets ( {} ). There are other ways, but you can specify the key value pairs with the key => value syntax.

Assign the hash variable called car_catalog to include the following car models and their showroom prices in dollars. Use the car model name as the hash key. The cars and prices are:

• Model: BMW Series 5, price: 100000
• Model: Mercedes 2000, price: 250000
• Model: Toyota Corolla, price: 20000
• Model: Lexus 3, price: 95000

For example: "I would like to buy one Lexus 3 for the price of 95000 Dollars."

Coding for Kids is an online interactive tutorial that teaches your kids how to code while playing!

Receive a 50% discount code by using the promo code:

Start now and play the first chapter for free, without signing up.

• Perl Basics
• Perl - Home
• Perl - Introduction
• Perl - Environment
• Perl - Syntax Overview
• Perl - Data Types

Perl - Variables

• Perl - Scalars
• Perl - Arrays
• Perl - Hashes
• Perl - IF...ELSE
• Perl - Loops
• Perl - Operators
• Perl - Date & Time
• Perl - Subroutines
• Perl - References
• Perl - Formats
• Perl - File I/O
• Perl - Directories
• Perl - Error Handling
• Perl - Special Variables
• Perl - Coding Standard
• Perl - Regular Expressions
• Perl - Sending Email
• Perl Advanced
• Perl - Socket Programming
• Perl - Object Oriented
• Perl - Database Access
• Perl - CGI Programming
• Perl - Packages & Modules
• Perl - Process Management
• Perl - Embedded Documentation
• Perl - Functions References
• Perl Useful Resources
• Perl - Questions and Answers
• Perl - Quick Guide
• Perl - Useful Resources
• Perl - Discussion
• Selected Reading
• UPSC IAS Exams Notes
• Developer's Best Practices
• Questions and Answers
• Effective Resume Writing
• HR Interview Questions
• Computer Glossary

Variables are the reserved memory locations to store values. This means that when you create a variable you reserve some space in memory.

Based on the data type of a variable, the interpreter allocates memory and decides what can be stored in the reserved memory. Therefore, by assigning different data types to variables, you can store integers, decimals, or strings in these variables.

We have learnt that Perl has the following three basic data types −

Accordingly, we are going to use three types of variables in Perl. A scalar variable will precede by a dollar sign ($) and it can store either a number, a string, or a reference. An array variable will precede by sign @ and it will store ordered lists of scalars. Finaly, the Hash variable will precede by sign % and will be used to store sets of key/value pairs.

Perl maintains every variable type in a separate namespace. So you can, without fear of conflict, use the same name for a scalar variable, an array, or a hash. This means that $foo and @foo are two different variables.

Creating Variables

Perl variables do not have to be explicitly declared to reserve memory space. The declaration happens automatically when you assign a value to a variable. The equal sign (=) is used to assign values to variables.

Keep a note that this is mandatory to declare a variable before we use it if we use use strict statement in our program.

The operand to the left of the = operator is the name of the variable, and the operand to the right of the = operator is the value stored in the variable. For example −

Here 25, "John Paul" and 1445.50 are the values assigned to $age , $name and $salary variables, respectively. Shortly we will see how we can assign values to arrays and hashes.

Scalar Variables

A scalar is a single unit of data. That data might be an integer number, floating point, a character, a string, a paragraph, or an entire web page. Simply saying it could be anything, but only a single thing.

Here is a simple example of using scalar variables −

This will produce the following result −

Array Variables

An array is a variable that stores an ordered list of scalar values. Array variables are preceded by an "at" (@) sign. To refer to a single element of an array, you will use the dollar sign ($) with the variable name followed by the index of the element in square brackets.

Here is a simple example of using array variables −

Here we used escape sign (\) before the $ sign just to print it. Other Perl will understand it as a variable and will print its value. When executed, this will produce the following result −

Hash Variables

A hash is a set of key/value pairs. Hash variables are preceded by a percent (%) sign. To refer to a single element of a hash, you will use the hash variable name followed by the "key" associated with the value in curly brackets.

Here is a simple example of using hash variables −

Variable Context

Perl treats same variable differently based on Context, i.e., situation where a variable is being used. Let's check the following example −

Here @names is an array, which has been used in two different contexts. First we copied it into anyother array, i.e., list, so it returned all the elements assuming that context is list context. Next we used the same array and tried to store this array in a scalar, so in this case it returned just the number of elements in this array assuming that context is scalar context. Following table lists down the various contexts −

Sr.No.	Context & Description
1	Assignment to a scalar variable evaluates the right-hand side in a scalar context.
2	Assignment to an array or a hash evaluates the right-hand side in a list context.
3	Boolean context is simply any place where an expression is being evaluated to see whether it's true or false.
4	This context not only doesn't care what the return value is, it doesn't even want a return value.
5	This context only happens inside quotes, or things that work like quotes.

Programming-Idioms

• C++
• Or search :

Idiom #249 Declare and assign multiple variables

Define variables a , b and c in a concise way. Explain if they need to have the same type.

• View revisions
• Report a bug

Please choose a nickname before doing this

No security, no password. Other people might choose the same nickname.

• 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.

Perl Monk, Perl Meditation

by (Friar)

) NODE.title = Multiple assignment in Perl NODE.owner = 813463 N.title = monktitlebar sitedoclet N.owner = 17342 -->

( = : , )

has asked for the wisdom of the Perl Monks concerning the following question: That is shorter version of the following assignment (I think they call it atomic version): How do I do the first assignment in Perl? My Perl version is Thank you. Multiple assignment in Perl or Code

Replies are listed 'Best First'.
by on Jul 22, 2018 at 23:36 UTC
by (Friar) on Jul 23, 2018 at 00:23 UTC
by (Canon) on Jul 23, 2018 at 04:34 UTC
in front of it? That would cause issues because it would declare two new variables within the loop which would mask the old variables. Also, and are really bad variable names to choose in Perl. There are special variables with these names used by so using them for other stuff can occasionally cause issues. and would be fine, as would more meaningful variable names.
by (Friar) on Jul 24, 2018 at 13:05 UTC
by (Canon) on Jul 24, 2018 at 06:13 UTC
" operator has a lower precedence than " ". Therefore, your version means which does, practically speaking, nothing. You have to add parens to enforce the proper precedence.

Back to Seekers of Perl Wisdom

Password:

www . com | www . net | www . org

• GrandFather
• Seekers of Perl Wisdom
• Cool Uses for Perl
• Meditations
• PerlMonks Discussion
• Categorized Q&A
• Obfuscated Code
• Perl Poetry
• PerlMonks FAQ
• Guide to the Monastery
• What's New at PerlMonks
• Voting/Experience System
• Other Info Sources
• Nodes You Wrote
• My Watched Nodes
• Super Search
• List Nodes By Users
• Newest Nodes
• Recently Active Threads
• Selected Best Nodes
• Worst Nodes
• Saints in our Book
• Random Node
• The St. Larry Wall Shrine
• Offering Plate
• Planet Perl
• Perl Weekly
• Perl Mongers
• Perl documentation

Too much JavaScript Not enough JavaScript Just the right amount of JavaScript Not enough WebPerl

Results (35 votes) . Check out past polls .

-->

	•	‥ 🛈The London Perl and Raku Workshop takes place on 26th Oct 2024. If your company depends on Perl, .

1. Variables and their Related Operators

• Installing and getting started with Perl
• The Hash-bang line, or how to make a Perl scripts executable on Linux
• Perl Editor
• How to get Help for Perl?
• Perl on the command line
• Core Perl documentation and CPAN module documentation
• POD - Plain Old Documentation
• Debugging Perl scripts
• Common Warnings and Error messages in Perl
• Prompt, read from STDIN, read from the keyboard in Perl
• Automatic string to number conversion or casting in Perl
• Conditional statements, using if, else, elsif in Perl
• Boolean values in Perl
• Numerical operators
• String operators: concatenation (.), repetition (x)
• undef, the initial value and the defined function of Perl
• Strings in Perl: quoted, interpolated and escaped

Here documents, or how to create multi-line strings in Perl

• Scalar variables
• Comparing scalars in Perl
• String functions: length, lc, uc, index, substr
• Number Guessing game
• Scope of variables in Perl
• Short-circuit in boolean expressions
• How to exit from a Perl script?
• Standard output, standard error and command line redirection
• Warning when something goes wrong
• What does die do?
• Writing to files with Perl
• Appending to files
• Open and read from text files
• Don't Open Files in the old way
• Reading and writing binary files in Perl
• EOF - End of file in Perl
• tell how far have we read a file
• seek - move the position in the filehandle in Perl
• slurp mode - reading a file in one step
• Perl for loop explained with examples
• Perl Arrays
• Processing command line arguments - @ARGV in Perl
• How to process command line arguments in Perl using Getopt::Long
• Advanced usage of Getopt::Long for accepting command line arguments
• Perl split - to cut up a string into pieces
• How to read a CSV file using Perl?
• The year of 19100
• Scalar and List context in Perl, the size of an array
• Reading from a file in scalar and list context
• STDIN in scalar and list context
• Sorting arrays in Perl
• Sorting mixed strings
• Unique values in an array in Perl
• Manipulating Perl arrays: shift, unshift, push, pop
• Reverse Polish Calculator in Perl using a stack
• Using a queue in Perl
• Reverse an array, a string or a number
• The ternary operator in Perl
• Loop controls: next, last, continue, break
• min, max, sum in Perl using List::Util
• qw - quote word
• Subroutines and functions in Perl
• Passing multiple parameters to a function in Perl
• Variable number of parameters in Perl subroutines
• Returning multiple values or a list from a subroutine in Perl
• Understanding recursive subroutines - traversing a directory tree
• Hashes in Perl
• Creating a hash from an array in Perl
• Perl hash in scalar and list context
• exists - check if a key exists in a hash
• delete an element from a hash
• How to sort a hash in Perl?
• Count the frequency of words in text using Perl
• Introduction to Regexes in Perl 5
• Regex character classes
• Regex: special character classes
• Perl 5 Regex Quantifiers
• trim - removing leading and trailing white spaces with Perl
• Perl 5 Regex Cheat sheet
• What are -e, -z, -s, -M, -A, -C, -r, -w, -x, -o, -f, -d , -l in Perl?
• Current working directory in Perl (cwd, pwd)
• Running external programs from Perl with system
• qx or backticks - running external command and capturing the output
• How to remove, copy or rename a file with Perl
• Reading the content of a directory
• Traversing the filesystem - using a queue
• Download and install Perl
• Installing a Perl Module from CPAN on Windows, Linux and Mac OSX
• How to change @INC to find Perl modules in non-standard locations
• How to add a relative directory to @INC
• How to replace a string in a file with Perl
• How to read an Excel file in Perl
• How to create an Excel file with Perl?
• Sending HTML e-mail using Email::Stuffer
• Perl/CGI script with Apache2
• JSON in Perl
• Simple Database access using Perl DBI and SQL
• Reading from LDAP in Perl using Net::LDAP
• Global symbol requires explicit package name
• Variable declaration in Perl
• Use of uninitialized value
• Barewords in Perl
• Name "main::x" used only once: possible typo at ...
• Unknown warnings category
• Can't use string (...) as an HASH ref while "strict refs" in use at ...
• Symbolic references in Perl
• Can't locate ... in @INC
• Scalar found where operator expected
• "my" variable masks earlier declaration in same scope
• Can't call method ... on unblessed reference
• Argument ... isn't numeric in numeric ...
• Can't locate object method "..." via package "1" (perhaps you forgot to load "1"?)
• Useless use of hash element in void context
• Useless use of private variable in void context
• readline() on closed filehandle in Perl
• Possible precedence issue with control flow operator
• Scalar value ... better written as ...
• substr outside of string at ...
• Have exceeded the maximum number of attempts (1000) to open temp file/dir
• Use of implicit split to @_ is deprecated ...
• Multi dimensional arrays in Perl
• Multi dimensional hashes in Perl
• Minimal requirement to build a sane CPAN package
• Statement modifiers: reversed if statements
• What is autovivification?
• Formatted printing in Perl using printf and sprintf

Non-interpolating here document

Interpolating here document, warning: exact end-string at the end, here documents and code indentation, using q or qq instead.

Published on 2013-06-30

Author: Gabor Szabo

• about the translations

• Trending Now
• Foundational Courses
• Data Science
• Practice Problem
• Machine Learning
• System Design
• DevOps Tutorial

Perl | Variables and its Types

The reserved memory locations to store values are the Variables. This means that creating a variable, reserves a space in memory. Data type of a variable helps the interpreter to allocate memory and decide what to be stored in the reserved memory. Therefore, variables can store integers, decimals, or strings with the assignment of different data types to the variables. Perl has the following three basic data types namely – 

Hence, three types of variables will be used in Perl. A scalar variable can store either a number, a string, or a reference and will precede by a dollar sign ($). An array variable will store ordered lists of scalars and precede by @ sign. The Hash variable will be used to store sets of key/value pairs and will precede by sign %.     

Perl variables need not to be declared explicitly to reserve memory space. Just like other programming languages, the operand to the left of the ‘=’ operator is basically the name of the variable, and the operand to the right of the ‘=’ operator is basically the value stored in the variable.  For example: 

$age = 40;  $name = “XYZ”;  $rollno = 22;  Here 40, “XYZ” and 22 are the values assigned to $age, $name and $roll no variables, respectively.   

A scalar is a single unit of data. It is possible that the data might be an integer number, floating point, a character, a string, a paragraph, or an entire web page. Here is an example of using scalar variables  

Output:  

Variable that stores an ordered list of scalar values is of array type. Variables of Array Datatype are preceded by an “at” (@) sign. The dollar sign ($) is used to refer a single element of an array with the variable name followed by the index of the element in square brackets.  Here is an example of how to use an array variable: 

Here we used ‘\’ before the ‘$’ sign just to print it as a statement. Otherwise Perl will by default understand it as a variable and will print the value stored in it. When executed, following result will be produced –  Output:  

A hash is a set of key/value pairs. Variables of the Hash type are preceded by a modulus (%) sign. Keys are used to refer to a single variable in the Hash. To access these elements, Hash variable name followed by the Key associated with the value is used in curly brackets.  Following is a simple example to show Hash Variable:  

Based on the Context, Perl treats the same variable differently i.e., situation where a variable is being used.  Example:   

Here @names is an array, which has been used in two different contexts. First, we copied it into any other array, i.e., list, so it returned all the elements assuming that context is list context. Next, the same array is tried to be stored in a scalar, which further returned just the number of elements in this array by default assuming it to be a scalar context. Following table displays the various contexts: 

S no.	Context & Description
1.	Assignment to a scalar variable evaluates the value on the right of ‘=’ in a scalar context.
2.	Assignment to an array or a hash evaluates the right of ‘=’ in a list context.
3.	A boolean context is simply a place where expression is evaluated just to check whether it is true or false.
4.	This context doesn’t care what the return value is, even it doesn’t want a return value.
5.	This context will only arise inside the quotes (“”), or with things that behave like quotes(“”).

Similar Reads

• perl-data-types

Please Login to comment...

• Best Smartwatches in 2024: Top Picks for Every Need
• Top Budgeting Apps in 2024
• 10 Best Parental Control App in 2024
• Top Language Learning Apps in 2024
• GeeksforGeeks Practice - Leading Online Coding Platform

Improve your Coding Skills with Practice

What kind of Experience do you want to share?

Perl Variables

Updated March 31, 2023

Definition of Perl Variables

Perl variables are the variables that is used to reserved memory to store values of declared variables, it means that at the time of creation or declaration of a variable in Perl we reserve some space of memory to this variable. The memory allocation is based on which data type we have used at the time to create new variables. We can assign a string, integer and decimal data types to the variables on that basis memory are allocated to the variables in Perl. Variables are the most important and useful in Perl.

Start Your Free Software Development Course

Web development, programming languages, Software testing & others

How to Declare Perl Variables?

The examples are as follows:

• There are three basic data types available in perl. Using this type we have to declare the variable as per our requirement.
• If we want to declare a single variable we need to use my package name before the declaration of variables.
• There are three types of variables available, the declaration of every type is different as per variables.
• My $age = 20; # this is integer type variable declaration.
• My $name = “ABC”; # this is the string type variable declaration.
• my $salary = 10000.25; # this is the floating-point type variable declaration.
• @ages = (20, 25, 30, 45, 40); # integer type variable array declaration.
• @names = (‘ABC’, ‘PQR’, ‘XYZ’); # String type variable array declaration.
• @salary = (10.1, 10.2, 10.3, 10.4, 10.5); # Floating point type variable array declaration.

Below is the example of declaring hash variables:

• Hash variable declaration starts with a % sign. %data = (‘AB’, 25, ‘PQ’, 30, ‘XY’, 35, ‘AB’, 45, ‘PQ’, 40);

How to Initialize?

Below are the example to initialize.

• We can initialize variables as per the types of variables that we have used. Below is the example of initializing the scalar type variable are as follows.
• We can initialize variables in single as well as in multiple lines. We have taken the example of below variable declaration.
• Multiple line variable initialization –
• Single line variable initialization –
• In the above example, we have initialized variables in a single line as well as in multiple lines. The output of variable initialization is the same using single as well as multiple line initialization in Perl.

Types of Perl Variables with Example

The variables are basically of three types.

There are different types of notations used in perl to declare and initialize a variable. Scalar is used to declare integer or string and it starts or precedes with a dollar ($) sign in Perl. We have defined the type of variables as per record which type of data we have used in a program.

• The below example shows a scalar type variable in perl. In the below example we have declared three variable age as an integer type, name as string type and salary as floating-point type.
• Scalar type variable is started with my package name and precedes with a dollar ($) sign.
• Scalar variables state that a single unit data.

• Array variable in perl is nothing but an ordered list of scalar variables. Array variables in perl start with @ sign.
• In the below example, we have declared one variable as five value of ages and one variable as three value of names.
• Initialization of an array variable we need to use a $ sign to display a single element of array variables.
• Below is the example of an array variable in perl is as follows.

• The hash variable in Perl nothing but a set of key or value pairs. Hash variables in Perl start with a % sign.
• In the below example, we have declared key pairs of names and ages in data variables.
• Initialization of a hash variable we need to use a $ sign to display a single element of hash variables.

Below is the example of a hash variable:

Recommended Articles

This is a guide to Perl Variables. Here we also discuss the Introduction and how to declare perl variables? along with different examples and its code implementation. You may also have a look at the following articles to learn more –

• Perl Comments
• File Handling in Perl

By signing up, you agree to our Terms of Use and Privacy Policy .

*Please provide your correct email id. Login details for this Free course will be emailed to you

Forgot Password?

This website or its third-party tools use cookies, which are necessary to its functioning and required to achieve the purposes illustrated in the cookie policy. By closing this banner, scrolling this page, clicking a link or continuing to browse otherwise, you agree to our Privacy Policy

Explore 1000+ varieties of Mock tests View more

Submit Next Question

Early-Bird Offer: ENROLL NOW

• Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers
• Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand
• OverflowAI GenAI features for Teams
• OverflowAPI Train & fine-tune LLMs
• Labs The future of collective knowledge sharing
• About the company Visit the blog

Collectives™ on Stack Overflow

Find centralized, trusted content and collaborate around the technologies you use most.

Q&A for work

Connect and share knowledge within a single location that is structured and easy to search.

Get early access and see previews of new features.

Match regex and assign results in single line of code

I want to be able to do a regex match on a variable and assign the results to the variable itself. What is the best way to do it?

I want to essentially combine lines 2 and 3 in a single line of code:

Is there a shorter/simpler way to do this? Am I missing something?

9 Answers 9

This works because

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, i.e. , ( $1 , $2 , $3 …).

and because my($variable) = ... (note the parentheses around the scalar) supplies list context to the match.

If the pattern fails to match, $variable gets the undefined value.

• To add to the description to the list context. The OoO is equivalent to my($foo) = ("bar box" =~ /(soap|box)/); –  vol7ron Commented Oct 17, 2012 at 16:03
• I'll also add one other example. If you want to prepend a string to the results in the same call: my($foo) = map ( "prefix_$_", @{["bar box" =~ /(soap|box)/)]} ); . You'd end up with "prefix_box" in this case. –  vol7ron Commented Oct 17, 2012 at 18:01

Why do you want it to be shorter? Does is really matter?

If you are worried about the variable name or doing this repeatedly, wrap the thing in a subroutine and forget about it:

However you implement it, the point of the subroutine is to make it reuseable so you give a particular set of lines a short name that stands in their place.

Several other answers mention a destructive substitution:

I tend to keep the original data around, and Perl v5.14 has an /r flag that leaves the original alone and returns a new string with the replacement (instead of the count of replacements):

• I prefer this answer's use of conditional assignment to the more upvoted answer of ($variable) = $variable =~ /(find).*/ for cases where you explicitly want that conditional assignment if and only if the match works. –  stevesliva Commented Oct 3, 2019 at 14:57

Well, you could say

• I'd rather use the variable definition separately for clarity's sake, but yes this is a good solution for quick searches. Thanks. –  user354606 Commented Sep 6, 2010 at 15:41

You can do substitution as:

$a is now "stack"

• There is a problem with this, It will only replace the original variable if the entire string is present in the search pattern. If I were to use, say, /(\w+)over/ for matching, this wouldn't work. –  user354606 Commented Sep 6, 2010 at 16:11
• The pattern is arbitrary, matches do not necessarily begin or end at a line or word boundary. In any case, thanks, I guess there isn't any obviously simple way to do it. My primary concern was that I have to do this often, and it looks repetitive. –  user354606 Commented Sep 6, 2010 at 17:08
• Stumbled on this 2 year old thread via Google, and I feel this is the correct answer. Just edit the regex to fit your needs: /.*(find something).*/$1/; –  Mintx Commented Aug 21, 2012 at 22:32
• What if you want to have two variables -- the matched pattern and the original –  QED Commented Apr 30, 2016 at 20:22

From Perl Cookbook 2nd ed 6.1 Copying and Substituting Simultaneously

I just assumed everyone did it this way, amazed that no one gave this answer.

• This is substitution ( s/// ), not a match ( m// ). That doesn't make it not useful, but it's why it wasn't among the original answers. –  stevesliva Commented Oct 3, 2019 at 14:49

Almost ....

You can combine the match and retrieve the matched value with a substitution.

AFAIK, You will always have to copy the value though, unless you do not care to clobber the original.

$variable1 now equals "overflow" .

Also, to amplify the accepted answer using the ternary operator to allow you to specify a default if there is no match:

• 1 Because this declares $match , this expands the most-upvoted answer as well, allowing replacement of undefined with *defaultValue* in one-line containing the variable declaration. –  stevesliva Commented Oct 3, 2019 at 14:56

Your Answer

Reminder: Answers generated by artificial intelligence tools are not allowed on Stack Overflow. Learn more

Sign up or log in

Post as a guest.

Required, but never shown

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy .

• The Overflow Blog
• Masked self-attention: How LLMs learn relationships between tokens
• Deedy Das: from coding at Meta, to search at Google, to investing with Anthropic
• Featured on Meta
• User activation: Learnings and opportunities
• Preventing unauthorized automated access to the network
• Should low-scoring meta questions no longer be hidden on the Meta.SO home...
• Feedback Requested: How do you use the tagged questions page?
• Announcing the new Staging Ground Reviewer Stats Widget

Hot Network Questions

• What is the mechanical equivalent of an electronic AND gate?
• In John 3:16, what is the significance of Jesus' distinction between the terms 'world' and 'everyone who believes' within the context?
• How can I draw the intersection of a plane with a dome tent?
• Sticky goo making it hard to open and close the main 200amp breaker
• Has Azerbaijan signed a contract to purchase JF-17s from Pakistan?
• Does the Rogue's Evasion cost a reaction?
• Literature reference for variance of the variance of the binomial proportion
• How do those car USB bluetooth dongles work?
• Can we solve the Sorites paradox with probability?
• Purpose of sleeve on sledge hammer handle
• Is there a name for this Fibonacci formula that uses hyperbolic trigonometry?
• Why was Z moved to the end of the alphabet when Zeta was near the beginning?
• Do we have volitional control over our level of skepticism?
• Problems regressing y on x/y?
• FORTRAN90 test suite for Project Euler
• Matter made of neutral charges does not radiate?
• Waiting girl's face
• Where is this NPC's voice coming from?
• How to resize faces while one edge stays rooted/level?
• On a glassed landmass, how long would it take for plants to grow?
• I want a smooth orthogonalization process
• Java class subset of C++ std::list with efficient std::list::sort()
• How should I ask permission for using tables or figures from published papers, in my own paper?
• What cameos did Stan Lee have in the DC universe?