Perl language structure: Difference between revisions

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== Basic syntax ==

In Perl, the minimal [["Hello, World!" program|Hello World]] program may be written as follows:

<syntaxhighlight lang="perl">

print "Hello, World!\n"

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<syntaxhighlight lang="perl">

#!/usr/bin/env perl

print "Hello, World!\n";

</syntaxhighlight>

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The second line in the canonical form includes a semicolon, which is used to separate statements in Perl. With only a single statement in a block or file, a separator is unnecessary, so it can be omitted from the minimal form of the program—or more generally from the final statement in any block or file. The canonical form includes it, because it is common to terminate every statement even when it is unnecessary to do so, as this makes editing easier: code can be added to, or moved away from, the end of a block or file without having to adjust semicolons.

Version 5.0110 of Perl introduces a <code>say</code> function that implicitly appends a newline character to its output, making the minimal "Hello World" program even shorter:

<syntaxhighlight lang="perl">

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! Description

|-

|[[Scalar / Variable (computing)|Scalar]]

|{{tt|$}}

|{{code|$foo}}

|A single value; it may be a number, a [[String (computer science)|string]], a filehandle, or a [[Reference (computer science)|reference]].

|-

|[[Array data type|Array]]

|{{tt|@}}

|{{code|@foo}}

|An ordered collection of scalars.

|-

|[[Associative array|Hash]]

|{{tt|%}}

|{{code|%foo}}

|A map from strings to scalars; the strings are called ''keys'', and the scalars are called ''values''. Also known as an ''associative array''.

|-

|[[FilehandleFile handle]]

|{{CNone|none}}

|{{code|$foo}} or {{code|FOO}}

|An opaque representation of an open file or other target for reading, writing, or both.

|-

|[[Subroutine]]

|{{tt|&}}

|{{code|&foo}}

|A piece of code that may be passed arguments, be executed, and return data.

|-

|[[Perl language structure#Typeglob values|Typeglob]]

|{{tt|*}}

|{{code|*foo}}

|The [[symbol table]] entry for all types with the name 'foo'.

|}

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</syntaxhighlight>

All other (non-zero evaluating) values evaluate to true. This includes the odd self-describing literal string of "0 but true", which in fact is 0 as a number, but true when used as a boolean. All non-numeric strings also have this property, but this particular string is truncated by Perl without a numeric warning. A less explicit but more conceptually portable version of this string is '{{mono|0E0}}' or '{{mono|0e0}}', which does not rely on characters being evaluated as 0, because '0E0' is literally zero times ten to the power zero. The empty hash <code>{}</code> is also true; in this context <code>{}</code> is not an empty block, because <code>perl -e 'print ref {}'</code> returns <code>HASH</code>.

Evaluated boolean expressions are also scalar values. The documentation does not promise which ''particular'' value of true or false is returned. Many boolean operators return 1 for true and the empty-string for false. The ''{{code|defined()''}} function determines whether a variable has any value set. In the above examples, ''{{code|defined($false)''}} is true for every value except ''{{code|undef''}}.

If either 1 or 0 are specifically needed, an explicit conversion can be done using the [[conditional operator]]:

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</syntaxhighlight>

Individual elements of a list are accessed by providing a numerical index in square brackets. The scalar [[sigil (computer programming)|sigil]] must be used. Sublists (array slices) can also be specified, using a range or list of numeric indices in brackets. The array sigil is used in this case. For example, <code>$month[3]</code> is <code>"April"</code> (the first element in an array has an index value of 0), and <code>@month[4..6]</code> is <code>("May", "June", "July")</code>.

===Hash values===

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</syntaxhighlight>

Individual values in a hash are accessed by providing the corresponding key, in curly braces. The <code>$</code> sigil identifies the accessed element as a scalar. For example, {{code|$favorite{joe} }} equals {{code|'red'}}. A hash can also be initialized by setting its values individually:

<syntaxhighlight lang="perl">

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Multiple elements may be accessed using the <code>@</code> sigil instead (identifying the result as a list). For example,

{{code|@favorite{'joe', 'sam'} }} equals {{code|('red', 'blue')}}.

===Filehandles===

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===Array functions===

The number of elements in an array can be determined either by evaluating the array in scalar context or with the help of the <code>$#</code> sigil. The latter gives the index of the last element in the array, not the number of elements. The expressions scalar({{code|@array}}) and (<code>$#array&nbsp;+&nbsp;1</code>) are equivalent.

===Hash functions===

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Up until the 5.10.0 release, there was no [[switch statement]] in Perl 5. From 5.10.0 onward, a multi-way branch statement called <code>given</code>/<code>when</code> is available, which takes the following form:

use v5.10; <u># must be present to import the new 5.10 functions</u>

given ( ''expr'' ) { when ( ''cond'' ) { … } default { … } }

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@x = either; # returns (1, 2)

</syntaxhighlight>

===Anonymous functions===

{{Excerpt|Examples of anonymous functions|Perl 5|subsections=yes}}

==Regular expressions==