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            <h1>perlintro</h1>


  <!--    -->
<ul><li><a href="#NAME">NAME</a><li><a href="#DESCRIPTION">DESCRIPTION</a><ul><li><a href="#What-is-Perl%3f">What is Perl?</a><li><a href="#Running-Perl-programs">Running Perl programs</a><li><a href="#Safety-net">Safety net</a><li><a href="#Basic-syntax-overview">Basic syntax overview</a><li><a href="#Perl-variable-types">Perl variable types</a><li><a href="#Variable-scoping">Variable scoping</a><li><a href="#Conditional-and-looping-constructs">Conditional and looping constructs</a><li><a href="#Builtin-operators-and-functions">Builtin operators and functions</a><li><a href="#Files-and-I%2fO">Files and I/O</a><li><a href="#Regular-expressions">Regular expressions</a><li><a href="#Writing-subroutines">Writing subroutines</a><li><a href="#OO-Perl">OO Perl</a><li><a href="#Using-Perl-modules">Using Perl modules</a></ul><li><a href="#AUTHOR">AUTHOR</a></ul><a name="NAME"></a><h1>NAME</h1>
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<p>perlintro -- a brief introduction and overview of Perl</p>
<a name="DESCRIPTION"></a><h1>DESCRIPTION</h1>
<p>This document is intended to give you a quick overview of the Perl
programming language, along with pointers to further documentation.  It
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is intended as a "bootstrap" guide for those who are new to the
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language, and provides just enough information for you to be able to
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read other peoples' Perl and understand roughly what it's doing, or
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write your own simple scripts.</p>
<p>This introductory document does not aim to be complete.  It does not
even aim to be entirely accurate.  In some cases perfection has been
sacrificed in the goal of getting the general idea across.  You are
<i>strongly</i> advised to follow this introduction with more information
from the full Perl manual, the table of contents to which can be found
in <a href="http://search.cpan.org/perldoc/perltoc">perltoc</a>.</p>
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<p>Throughout this document you'll see references to other parts of the
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Perl documentation.  You can read that documentation using the <code class="inline"><span class="w">perldoc</span></code>
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command or whatever method you're using to read this document.</p>
<p>Throughout Perl's documentation, you'll find numerous examples intended
to help explain the discussed features.  Please keep in mind that many
of them are code fragments rather than complete programs.</p>
<p>These examples often reflect the style and preference of the author of
that piece of the documentation, and may be briefer than a corresponding
line of code in a real program.  Except where otherwise noted, you
should assume that <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">strict</span></code>
 and <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">warnings</span></code>
 statements
appear earlier in the "program", and that any variables used have
already been declared, even if those declarations have been omitted
to make the example easier to read.</p>
<p>Do note that the examples have been written by many different authors over
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a period of several decades.  Styles and techniques will therefore differ,
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although some effort has been made to not vary styles too widely in the
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same sections.  Do not consider one style to be better than others - "There's
More Than One Way To Do It" is one of Perl's mottos.  After all, in your
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journey as a programmer, you are likely to encounter different styles.</p>
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<a name="What-is-Perl%3f"></a><h2>What is Perl?</h2>
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<p>Perl is a general-purpose programming language originally developed for
text manipulation and now used for a wide range of tasks including
system administration, web development, network programming, GUI
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development, and more.</p>
<p>The language is intended to be practical (easy to use, efficient,
complete) rather than beautiful (tiny, elegant, minimal).  Its major
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features are that it's easy to use, supports both procedural and
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object-oriented (OO) programming, has powerful built-in support for text
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processing, and has one of the world's most impressive collections of
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third-party modules.</p>
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<p>Different definitions of Perl are given in <a href="perl.html">perl</a>, <a href="perlfaq1.html">perlfaq1</a> and
no doubt other places.  From this we can determine that Perl is different
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things to different people, but that lots of people think it's at least
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worth writing about.</p>
<a name="Running-Perl-programs"></a><h2>Running Perl programs</h2>
<p>To run a Perl program from the Unix command line:</p>
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<pre class="verbatim"><ol><li> <span class="w">perl</span> <span class="w">progname</span>.<span class="w">pl</span></li></ol></pre><p>Alternatively, put this as the first line of your script:</p>
<pre class="verbatim"><ol><li> <span class="c">#!/usr/bin/env perl</span></li></ol></pre><p>... and run the script as <i>/path/to/script.pl</i>.  Of course, it'll need
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to be executable first, so <code class="inline">chmod 755 script.pl</code> (under Unix).</p>
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<p>(This start line assumes you have the <b>env</b> program.  You can also put
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directly the path to your perl executable, like in <code class="inline">#!/usr/bin/perl</code>
).</p>
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<p>For more information, including instructions for other platforms such as
Windows and Mac OS, read <a href="perlrun.html">perlrun</a>.</p>
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<a name="Safety-net"></a><h2>Safety net</h2>
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<p>Perl by default is very forgiving.  In order to make it more robust
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it is recommended to start every program with the following lines:</p>
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<pre class="verbatim"><ol><li> <span class="c">#!/usr/bin/perl</span></li><li> <a class="l_k" href="functions/use.html">use</a> <span class="w">strict</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/use.html">use</a> <span class="w">warnings</span><span class="sc">;</span></li></ol></pre><p>The two additional lines request from perl to catch various common
problems in your code.  They check different things so you need both.  A
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potential problem caught by <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">strict</span><span class="sc">;</span></code>
 will cause your code to stop
immediately when it is encountered, while <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">warnings</span><span class="sc">;</span></code>
 will merely
give a warning (like the command-line switch <b>-w</b>) and let your code run.
To read more about them check their respective manual pages at <a href="strict.html">strict</a>
and <a href="warnings.html">warnings</a>.</p>
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<a name="Basic-syntax-overview"></a><h2>Basic syntax overview</h2>
<p>A Perl script or program consists of one or more statements.  These
statements are simply written in the script in a straightforward
fashion.  There is no need to have a <code class="inline"><span class="i">main</span><span class="s">(</span><span class="s">)</span></code>
 function or anything of
that kind.</p>
<p>Perl statements end in a semi-colon:</p>
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<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Hello, world&quot;</span><span class="sc">;</span></li></ol></pre><p>Comments start with a hash symbol and run to the end of the line</p>
<pre class="verbatim"><ol><li> <span class="c"># This is a comment</span></li></ol></pre><p>Whitespace is irrelevant:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a></li><li>     <span class="q">&quot;Hello, world&quot;</span></li><li>     <span class="sc">;</span></li></ol></pre><p>... except inside quoted strings:</p>
<pre class="verbatim"><ol><li> <span class="c"># this would print with a linebreak in the middle</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Hello</span></li><li> <span class="q"> world&quot;</span><span class="sc">;</span></li></ol></pre><p>Double quotes or single quotes may be used around literal strings:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Hello, world&quot;</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&#39;Hello, world&#39;</span><span class="sc">;</span></li></ol></pre><p>However, only double quotes "interpolate" variables and special
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characters such as newlines (<code class="inline">\<span class="w">n</span></code>
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):</p>
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<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Hello, $name\n&quot;</span><span class="sc">;</span>     <span class="c"># works fine</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&#39;Hello, $name\n&#39;</span><span class="sc">;</span>     <span class="c"># prints $name\n literally</span></li></ol></pre><p>Numbers don't need quotes around them:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a> <span class="n">42</span><span class="sc">;</span></li></ol></pre><p>You can use parentheses for functions' arguments or omit them
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according to your personal taste.  They are only required
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occasionally to clarify issues of precedence.</p>
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<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a><span class="s">(</span><span class="q">&quot;Hello, world\n&quot;</span><span class="s">)</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Hello, world\n&quot;</span><span class="sc">;</span></li></ol></pre><p>More detailed information about Perl syntax can be found in <a href="perlsyn.html">perlsyn</a>.</p>
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<a name="Perl-variable-types"></a><h2>Perl variable types</h2>
<p>Perl has three main variable types: scalars, arrays, and hashes.</p>
<ul>
<li><a name="Scalars"></a><b>Scalars</b>
<p>A scalar represents a single value:</p>
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<pre class="verbatim"><ol><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">$animal</span> = <span class="q">&quot;camel&quot;</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">$answer</span> = <span class="n">42</span><span class="sc">;</span></li></ol></pre><p>Scalar values can be strings, integers or floating point numbers, and Perl
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will automatically convert between them as required.  There is no need
to pre-declare your variable types, but you have to declare them using
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the <code class="inline"><a class="l_k" href="functions/my.html">my</a></code> keyword the first time you use them.  (This is one of the
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requirements of <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">strict</span><span class="sc">;</span></code>
.)</p>
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<p>Scalar values can be used in various ways:</p>
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<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">$animal</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;The animal is $animal\n&quot;</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;The square of $answer is &quot;</span><span class="cm">,</span> <span class="i">$answer</span> * <span class="i">$answer</span><span class="cm">,</span> <span class="q">&quot;\n&quot;</span><span class="sc">;</span></li></ol></pre><p>There are a number of "magic" scalars with names that look like
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punctuation or line noise.  These special variables are used for all
kinds of purposes, and are documented in <a href="perlvar.html">perlvar</a>.  The only one you
need to know about for now is <code class="inline"><span class="i">$_</span></code>
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 which is the "default variable".
It's used as the default argument to a number of functions in Perl, and
it's set implicitly by certain looping constructs.</p>
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<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a><span class="sc">;</span>          <span class="c"># prints contents of $_ by default</span></li></ol></pre></li>
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<li><a name="Arrays"></a><b>Arrays</b>
<p>An array represents a list of values:</p>
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<pre class="verbatim"><ol><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">@animals</span> = <span class="s">(</span><span class="q">&quot;camel&quot;</span><span class="cm">,</span> <span class="q">&quot;llama&quot;</span><span class="cm">,</span> <span class="q">&quot;owl&quot;</span><span class="s">)</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">@numbers</span> = <span class="s">(</span><span class="n">23</span><span class="cm">,</span> <span class="n">42</span><span class="cm">,</span> <span class="n">69</span><span class="s">)</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">@mixed</span>   = <span class="s">(</span><span class="q">&quot;camel&quot;</span><span class="cm">,</span> <span class="n">42</span><span class="cm">,</span> <span class="n">1.23</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>Arrays are zero-indexed.  Here's how you get at elements in an array:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">$animals</span>[<span class="n">0</span>]<span class="sc">;</span>              <span class="c"># prints &quot;camel&quot;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">$animals</span>[<span class="n">1</span>]<span class="sc">;</span>              <span class="c"># prints &quot;llama&quot;</span></li></ol></pre><p>The special variable <code class="inline"><span class="i">$#array</span></code>
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 tells you the index of the last element
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of an array:</p>
503
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">$mixed</span>[<span class="i">$#mixed</span>]<span class="sc">;</span>       <span class="c"># last element, prints 1.23</span></li></ol></pre><p>You might be tempted to use <code class="inline"><span class="i">$#array</span> + <span class="n">1</span></code>
504
 to tell you how many items there
505
are in an array.  Don't bother.  As it happens, using <code class="inline"><span class="i">@array</span></code>
506
 where Perl
507
expects to find a scalar value ("in scalar context") will give you the number
508
of elements in the array:</p>
509
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="i">@animals</span> &lt; <span class="n">5</span><span class="s">)</span> <span class="s">{</span> ... <span class="s">}</span></li></ol></pre><p>The elements we're getting from the array start with a <code class="inline"><span class="i">$</span></code>
510
 because
511
we're getting just a single value out of the array; you ask for a scalar,
512 513
you get a scalar.</p>
<p>To get multiple values from an array:</p>
514
<pre class="verbatim"><ol><li> <span class="i">@animals</span>[<span class="n">0</span><span class="cm">,</span><span class="n">1</span>]<span class="sc">;</span>                 <span class="c"># gives (&quot;camel&quot;, &quot;llama&quot;);</span></li><li> <span class="i">@animals</span>[<span class="n">0</span>..<span class="n">2</span>]<span class="sc">;</span>                <span class="c"># gives (&quot;camel&quot;, &quot;llama&quot;, &quot;owl&quot;);</span></li><li> <span class="i">@animals</span>[<span class="n">1</span>..<span class="i">$#animals</span>]<span class="sc">;</span>        <span class="c"># gives all except the first element</span></li></ol></pre><p>This is called an "array slice".</p>
515
<p>You can do various useful things to lists:</p>
516
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">@sorted</span>    = <a class="l_k" href="functions/sort.html">sort</a> <span class="i">@animals</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">@backwards</span> = <a class="l_k" href="functions/reverse.html">reverse</a> <span class="i">@numbers</span><span class="sc">;</span></li></ol></pre><p>There are a couple of special arrays too, such as <code class="inline"><span class="i">@ARGV</span></code>
517 518 519 520 521 522 523
 (the command
line arguments to your script) and <code class="inline"><span class="i">@_</span></code>
 (the arguments passed to a
subroutine).  These are documented in <a href="perlvar.html">perlvar</a>.</p>
</li>
<li><a name="Hashes"></a><b>Hashes</b>
<p>A hash represents a set of key/value pairs:</p>
524
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">%fruit_color</span> = <span class="s">(</span><span class="q">&quot;apple&quot;</span><span class="cm">,</span> <span class="q">&quot;red&quot;</span><span class="cm">,</span> <span class="q">&quot;banana&quot;</span><span class="cm">,</span> <span class="q">&quot;yellow&quot;</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>You can use whitespace and the <code class="inline"><span class="cm">=&gt;</span></code>
525 526
 operator to lay them out more
nicely:</p>
527 528
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">%fruit_color</span> = <span class="s">(</span></li><li>     <span class="w">apple</span>  <span class="cm">=&gt;</span> <span class="q">&quot;red&quot;</span><span class="cm">,</span></li><li>     <span class="w">banana</span> <span class="cm">=&gt;</span> <span class="q">&quot;yellow&quot;</span><span class="cm">,</span></li><li> <span class="s">)</span><span class="sc">;</span></li></ol></pre><p>To get at hash elements:</p>
<pre class="verbatim"><ol><li> <span class="i">$fruit_color</span>{<span class="q">&quot;apple&quot;</span>}<span class="sc">;</span>           <span class="c"># gives &quot;red&quot;</span></li></ol></pre><p>You can get at lists of keys and values with <code class="inline"><a class="l_k" href="functions/keys.html">keys()</a></code> and
529
<code class="inline"><a class="l_k" href="functions/values.html">values()</a></code>.</p>
530
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">@fruits</span> = <a class="l_k" href="functions/keys.html">keys</a> <span class="i">%fruit_colors</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">@colors</span> = <a class="l_k" href="functions/values.html">values</a> <span class="i">%fruit_colors</span><span class="sc">;</span></li></ol></pre><p>Hashes have no particular internal order, though you can sort the keys
531
and loop through them.</p>
532
<p>Just like special scalars and arrays, there are also special hashes.
533 534 535 536 537 538 539 540 541 542
The most well known of these is <code class="inline"><span class="i">%ENV</span></code>
 which contains environment
variables.  Read all about it (and other special variables) in
<a href="perlvar.html">perlvar</a>.</p>
</li>
</ul>
<p>Scalars, arrays and hashes are documented more fully in <a href="perldata.html">perldata</a>.</p>
<p>More complex data types can be constructed using references, which allow
you to build lists and hashes within lists and hashes.</p>
<p>A reference is a scalar value and can refer to any other Perl data
543
type.  So by storing a reference as the value of an array or hash
544
element, you can easily create lists and hashes within lists and
545
hashes.  The following example shows a 2 level hash of hash
546
structure using anonymous hash references.</p>
547
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">$variables</span> = <span class="s">{</span></li><li>     <span class="w">scalar</span>  <span class="cm">=&gt;</span>  <span class="s">{</span></li><li>                  <span class="w">description</span> <span class="cm">=&gt;</span> <span class="q">&quot;single item&quot;</span><span class="cm">,</span></li><li>                  <span class="w">sigil</span> <span class="cm">=&gt;</span> <span class="q">&#39;$&#39;</span><span class="cm">,</span></li><li>                 <span class="s">}</span><span class="cm">,</span></li><li>     <span class="w">array</span>   <span class="cm">=&gt;</span>  <span class="s">{</span></li><li>                  <span class="w">description</span> <span class="cm">=&gt;</span> <span class="q">&quot;ordered list of items&quot;</span><span class="cm">,</span></li><li>                  <span class="w">sigil</span> <span class="cm">=&gt;</span> <span class="q">&#39;@&#39;</span><span class="cm">,</span></li><li>                 <span class="s">}</span><span class="cm">,</span></li><li>     <span class="w">hash</span>    <span class="cm">=&gt;</span>  <span class="s">{</span></li><li>                  <span class="w">description</span> <span class="cm">=&gt;</span> <span class="q">&quot;key/value pairs&quot;</span><span class="cm">,</span></li><li>                  <span class="w">sigil</span> <span class="cm">=&gt;</span> <span class="q">&#39;%&#39;</span><span class="cm">,</span></li><li>                 <span class="s">}</span><span class="cm">,</span></li><li> <span class="s">}</span><span class="sc">;</span></li><li></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Scalars begin with a $variables-&gt;{&#39;scalar&#39;}-&gt;{&#39;sigil&#39;}\n&quot;</span><span class="sc">;</span></li></ol></pre><p>Exhaustive information on the topic of references can be found in
548 549 550
<a href="perlreftut.html">perlreftut</a>, <a href="perllol.html">perllol</a>, <a href="perlref.html">perlref</a> and <a href="perldsc.html">perldsc</a>.</p>
<a name="Variable-scoping"></a><h2>Variable scoping</h2>
<p>Throughout the previous section all the examples have used the syntax:</p>
551 552
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">$var</span> = <span class="q">&quot;value&quot;</span><span class="sc">;</span></li></ol></pre><p>The <code class="inline"><a class="l_k" href="functions/my.html">my</a></code> is actually not required; you could just use:</p>
<pre class="verbatim"><ol><li> <span class="i">$var</span> = <span class="q">&quot;value&quot;</span><span class="sc">;</span></li></ol></pre><p>However, the above usage will create global variables throughout your
553 554 555 556
program, which is bad programming practice.  <code class="inline"><a class="l_k" href="functions/my.html">my</a></code> creates lexically
scoped variables instead.  The variables are scoped to the block
(i.e. a bunch of statements surrounded by curly-braces) in which they
are defined.</p>
557
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">$x</span> = <span class="q">&quot;foo&quot;</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">$some_condition</span> = <span class="n">1</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="i">$some_condition</span><span class="s">)</span> <span class="s">{</span></li><li>     <a class="l_k" href="functions/my.html">my</a> <span class="i">$y</span> = <span class="q">&quot;bar&quot;</span><span class="sc">;</span></li><li>     <a class="l_k" href="functions/print.html">print</a> <span class="i">$x</span><span class="sc">;</span>           <span class="c"># prints &quot;foo&quot;</span></li><li>     <a class="l_k" href="functions/print.html">print</a> <span class="i">$y</span><span class="sc">;</span>           <span class="c"># prints &quot;bar&quot;</span></li><li> <span class="s">}</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">$x</span><span class="sc">;</span>               <span class="c"># prints &quot;foo&quot;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">$y</span><span class="sc">;</span>               <span class="c"># prints nothing; $y has fallen out of scope</span></li></ol></pre><p>Using <code class="inline"><a class="l_k" href="functions/my.html">my</a></code> in combination with a <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">strict</span><span class="sc">;</span></code>
558
 at the top of
559
your Perl scripts means that the interpreter will pick up certain common
560
programming errors.  For instance, in the example above, the final
561
<code class="inline"><a class="l_k" href="functions/print.html">print</a> <span class="i">$y</span></code>
562
 would cause a compile-time error and prevent you from
563 564
running the program.  Using <code class="inline"><span class="w">strict</span></code>
 is highly recommended.</p>
565
<a name="Conditional-and-looping-constructs"></a><h2>Conditional and looping constructs</h2>
566 567 568 569
<p>Perl has most of the usual conditional and looping constructs.  As of Perl
5.10, it even has a case/switch statement (spelled <code class="inline">given</code>
/<code class="inline">when</code>
).  See
570
<a href="perlsyn.html#Switch-Statements">Switch Statements in perlsyn</a> for more details.</p>
571
<p>The conditions can be any Perl expression.  See the list of operators in
572
the next section for information on comparison and boolean logic operators,
573 574 575
which are commonly used in conditional statements.</p>
<ul>
<li><a name="if"></a><b>if</b>
576 577
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span> <span class="w">condition</span> <span class="s">)</span> <span class="s">{</span></li><li>     ...</li><li> <span class="s">}</span> <a class="l_k" href="functions/elsif.html">elsif</a> <span class="s">(</span> <span class="w">other</span> <span class="w">condition</span> <span class="s">)</span> <span class="s">{</span></li><li>     ...</li><li> <span class="s">}</span> <a class="l_k" href="functions/else.html">else</a> <span class="s">{</span></li><li>     ...</li><li> <span class="s">}</span></li></ol></pre><p>There's also a negated version of it:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/unless.html">unless</a> <span class="s">(</span> <span class="w">condition</span> <span class="s">)</span> <span class="s">{</span></li><li>     ...</li><li> <span class="s">}</span></li></ol></pre><p>This is provided as a more readable version of <code class="inline">if (!<i>condition</i>)</code>.</p>
578
<p>Note that the braces are required in Perl, even if you've only got one
579 580
line in the block.  However, there is a clever way of making your one-line
conditional blocks more English like:</p>
581
<pre class="verbatim"><ol><li> <span class="c"># the traditional way</span></li><li> <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="i">$zippy</span><span class="s">)</span> <span class="s">{</span></li><li>     <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Yow!&quot;</span><span class="sc">;</span></li><li> <span class="s">}</span></li><li></li><li> <span class="c"># the Perlish post-condition way</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Yow!&quot;</span> <a class="l_k" href="functions/if.html">if</a> <span class="i">$zippy</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;We have no bananas&quot;</span> <a class="l_k" href="functions/unless.html">unless</a> <span class="i">$bananas</span><span class="sc">;</span></li></ol></pre></li>
582
<li><a name="while"></a><b>while</b>
583
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/while.html">while</a> <span class="s">(</span> <span class="w">condition</span> <span class="s">)</span> <span class="s">{</span></li><li>     ...</li><li> <span class="s">}</span></li></ol></pre><p>There's also a negated version, for the same reason we have <code class="inline">unless</code>
584
:</p>
585
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/until.html">until</a> <span class="s">(</span> <span class="w">condition</span> <span class="s">)</span> <span class="s">{</span></li><li>     ...</li><li> <span class="s">}</span></li></ol></pre><p>You can also use <code class="inline">while</code>
586
 in a post-condition:</p>
587
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;LA LA LA\n&quot;</span> <a class="l_k" href="functions/while.html">while</a> <span class="n">1</span><span class="sc">;</span>          <span class="c"># loops forever</span></li></ol></pre></li>
588 589
<li><a name="for"></a><b>for</b>
<p>Exactly like C:</p>
590
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/for.html">for</a> <span class="s">(</span><span class="i">$i</span> = <span class="n">0</span><span class="sc">;</span> <span class="i">$i</span> &lt;= <span class="i">$max</span><span class="sc">;</span> <span class="i">$i</span>++<span class="s">)</span> <span class="s">{</span></li><li>     ...</li><li> <span class="s">}</span></li></ol></pre><p>The C style for loop is rarely needed in Perl since Perl provides
591 592 593 594
the more friendly list scanning <code class="inline">foreach</code>
 loop.</p>
</li>
<li><a name="foreach"></a><b>foreach</b>
595 596 597 598 599
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/foreach.html">foreach</a> <span class="s">(</span><span class="i">@array</span><span class="s">)</span> <span class="s">{</span></li><li>     <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;This element is $_\n&quot;</span><span class="sc">;</span></li><li> <span class="s">}</span></li><li></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">$list</span>[<span class="i">$_</span>] <a class="l_k" href="functions/foreach.html">foreach</a> <span class="n">0</span> .. <span class="i">$max</span><span class="sc">;</span></li><li></li><li> <span class="c"># you don&#39;t have to use the default $_ either...</span></li><li> <a class="l_k" href="functions/foreach.html">foreach</a> <a class="l_k" href="functions/my.html">my</a> <span class="i">$key</span> <span class="s">(</span><a class="l_k" href="functions/keys.html">keys</a> <span class="i">%hash</span><span class="s">)</span> <span class="s">{</span></li><li>     <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;The value of $key is $hash{$key}\n&quot;</span><span class="sc">;</span></li><li> <span class="s">}</span></li></ol></pre><p>The <code class="inline">foreach</code>
 keyword is actually a synonym for the <code class="inline">for</code>

keyword.  See <code class="inline"><a href="perlsyn.html#Foreach-Loops">Foreach Loops in perlsyn</a></code>.</p>
</li>
600
</ul>
601
<p>For more detail on looping constructs (and some that weren't mentioned in
602 603 604
this overview) see <a href="perlsyn.html">perlsyn</a>.</p>
<a name="Builtin-operators-and-functions"></a><h2>Builtin operators and functions</h2>
<p>Perl comes with a wide selection of builtin functions.  Some of the ones
605
we've already seen include <code class="inline"><a class="l_k" href="functions/print.html">print</a></code>, <code class="inline"><a class="l_k" href="functions/sort.html">sort</a></code> and <code class="inline"><a class="l_k" href="functions/reverse.html">reverse</a></code>.  A list of
606
them is given at the start of <a href="perlfunc.html">perlfunc</a> and you can easily read
607
about any given function by using <code class="inline">perldoc -f <i>functionname</i></code>.</p>
608 609 610 611
<p>Perl operators are documented in full in <a href="perlop.html">perlop</a>, but here are a few
of the most common ones:</p>
<ul>
<li><a name="Arithmetic"></a><b>Arithmetic</b>
612
<pre class="verbatim"><ol><li> +   <span class="w">addition</span></li><li> -   <span class="w">subtraction</span></li><li> *   <span class="w">multiplication</span></li><li> /   <span class="w">division</span></li></ol></pre></li>
613
<li><a name="Numeric-comparison"></a><b>Numeric comparison</b>
614
<pre class="verbatim"><ol><li> ==  <span class="w">equality</span></li><li> !=  <span class="w">inequality</span></li><li> &lt;   <span class="w">less</span> <span class="w">than</span></li><li> &gt;   <span class="w">greater</span> <span class="w">than</span></li><li> &lt;=  <span class="w">less</span> <span class="w">than</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">equal</span></li><li> &gt;=  <span class="w">greater</span> <span class="w">than</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">equal</span></li></ol></pre></li>
615
<li><a name="String-comparison"></a><b>String comparison</b>
616
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/eq.html">eq</a>  <span class="w">equality</span></li><li> <a class="l_k" href="functions/ne.html">ne</a>  <span class="w">inequality</span></li><li> <a class="l_k" href="functions/lt.html">lt</a>  <span class="w">less</span> <span class="w">than</span></li><li> <a class="l_k" href="functions/gt.html">gt</a>  <span class="w">greater</span> <span class="w">than</span></li><li> <a class="l_k" href="functions/le.html">le</a>  <span class="w">less</span> <span class="w">than</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">equal</span></li><li> <a class="l_k" href="functions/ge.html">ge</a>  <span class="w">greater</span> <span class="w">than</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">equal</span></li></ol></pre><p>(Why do we have separate numeric and string comparisons?  Because we don't
617
have special variable types, and Perl needs to know whether to sort
618 619 620 621
numerically (where 99 is less than 100) or alphabetically (where 100 comes
before 99).</p>
</li>
<li><a name="Boolean-logic"></a><b>Boolean logic</b>
622
<pre class="verbatim"><ol><li> &amp;&amp;  <a class="l_k" href="functions/and.html">and</a></li><li> ||  <a class="l_k" href="functions/or.html">or</a></li><li> !   <a class="l_k" href="functions/not.html">not</a></li></ol></pre><p>(<code class="inline">and</code>
623 624
, <code class="inline">or</code>
 and <code class="inline">not</code>
625
 aren't just in the above table as descriptions
626
of the operators.  They're also supported as operators in their own
627
right.  They're more readable than the C-style operators, but have
628
different precedence to <code class="inline">&amp;&amp;</code> and friends.  Check <a href="perlop.html">perlop</a> for more
629 630 631
detail.)</p>
</li>
<li><a name="Miscellaneous"></a><b>Miscellaneous</b>
632
<pre class="verbatim"><ol><li> =   <span class="w">assignment</span></li><li> .   <span class="w">string</span> <span class="w">concatenation</span></li><li> <span class="w">x</span>   <span class="w">string</span> <span class="w">multiplication</span></li><li> ..  <span class="w">range</span> <span class="w">operator</span> <span class="s">(</span><span class="w">creates</span> <span class="w">a</span> <span class="w">list</span> <span class="w">of</span> <span class="w">numbers</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">strings</span><span class="s">)</span></li></ol></pre></li>
633 634 635
</ul>
<p>Many operators can be combined with a <code class="inline">=</code>
 as follows:</p>
636
<pre class="verbatim"><ol><li> <span class="i">$a</span> += <span class="n">1</span><span class="sc">;</span>        <span class="c"># same as $a = $a + 1</span></li><li> <span class="i">$a</span> -= <span class="n">1</span><span class="sc">;</span>        <span class="c"># same as $a = $a - 1</span></li><li> <span class="i">$a</span> .= <span class="q">&quot;\n&quot;</span><span class="sc">;</span>     <span class="c"># same as $a = $a . &quot;\n&quot;;</span></li></ol></pre><a name="Files-and-I%2fO"></a><h2>Files and I/O</h2>
637
<p>You can open a file for input or output using the <code class="inline"><a class="l_k" href="functions/open.html">open()</a></code> function.
638
It's documented in extravagant detail in <a href="perlfunc.html">perlfunc</a> and <a href="perlopentut.html">perlopentut</a>,
639
but in short:</p>
640
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/open.html">open</a><span class="s">(</span><a class="l_k" href="functions/my.html">my</a> <span class="i">$in</span><span class="cm">,</span>  <span class="q">&quot;&lt;&quot;</span><span class="cm">,</span>  <span class="q">&quot;input.txt&quot;</span><span class="s">)</span>  <a class="l_k" href="functions/or.html">or</a> <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;Can&#39;t open input.txt: $!&quot;</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/open.html">open</a><span class="s">(</span><a class="l_k" href="functions/my.html">my</a> <span class="i">$out</span><span class="cm">,</span> <span class="q">&quot;&gt;&quot;</span><span class="cm">,</span>  <span class="q">&quot;output.txt&quot;</span><span class="s">)</span> <a class="l_k" href="functions/or.html">or</a> <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;Can&#39;t open output.txt: $!&quot;</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/open.html">open</a><span class="s">(</span><a class="l_k" href="functions/my.html">my</a> <span class="i">$log</span><span class="cm">,</span> <span class="q">&quot;&gt;&gt;&quot;</span><span class="cm">,</span> <span class="q">&quot;my.log&quot;</span><span class="s">)</span>     <a class="l_k" href="functions/or.html">or</a> <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;Can&#39;t open my.log: $!&quot;</span><span class="sc">;</span></li></ol></pre><p>You can read from an open filehandle using the <code class="inline">&lt;&gt;</code>
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 operator.  In
scalar context it reads a single line from the filehandle, and in list
context it reads the whole file in, assigning each line to an element of
the list:</p>
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<pre class="verbatim"><ol><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">$line</span>  = <span class="q">&lt;$in&gt;</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">@lines</span> = <span class="q">&lt;$in&gt;</span><span class="sc">;</span></li></ol></pre><p>Reading in the whole file at one time is called slurping.  It can
be useful but it may be a memory hog.  Most text file processing
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can be done a line at a time with Perl's looping constructs.</p>
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<p>The <code class="inline">&lt;&gt;</code>
 operator is most often seen in a <code class="inline">while</code>
 loop:</p>
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<pre class="verbatim"><ol><li> <a class="l_k" href="functions/while.html">while</a> <span class="s">(</span><span class="q">&lt;$in&gt;</span><span class="s">)</span> <span class="s">{</span>     <span class="c"># assigns each line in turn to $_</span></li><li>     <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Just read in this line: $_&quot;</span><span class="sc">;</span></li><li> <span class="s">}</span></li></ol></pre><p>We've already seen how to print to standard output using <code class="inline"><a class="l_k" href="functions/print.html">print()</a></code>.
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However, <code class="inline"><a class="l_k" href="functions/print.html">print()</a></code> can also take an optional first argument specifying
which filehandle to print to:</p>
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<pre class="verbatim"><ol><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">STDERR</span> <span class="q">&quot;This is your final warning.\n&quot;</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">$out</span> <span class="i">$record</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/print.html">print</a> <span class="i">$log</span> <span class="i">$logmessage</span><span class="sc">;</span></li></ol></pre><p>When you're done with your filehandles, you should <code class="inline"><a class="l_k" href="functions/close.html">close()</a></code> them
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(though to be honest, Perl will clean up after you if you forget):</p>
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<pre class="verbatim"><ol><li> <a class="l_k" href="functions/close.html">close</a> <span class="i">$in</span> <a class="l_k" href="functions/or.html">or</a> <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;$in: $!&quot;</span><span class="sc">;</span></li></ol></pre><a name="Regular-expressions"></a><h2>Regular expressions</h2>
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<p>Perl's regular expression support is both broad and deep, and is the
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subject of lengthy documentation in <a href="perlrequick.html">perlrequick</a>, <a href="perlretut.html">perlretut</a>, and
elsewhere.  However, in short:</p>
<ul>
<li><a name="Simple-matching"></a><b>Simple matching</b>
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<pre class="verbatim"><ol><li> <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="q">/foo/</span><span class="s">)</span>       <span class="s">{</span> ... <span class="s">}</span>  <span class="c"># true if $_ contains &quot;foo&quot;</span></li><li> <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="i">$a</span> =~ <span class="q">/foo/</span><span class="s">)</span> <span class="s">{</span> ... <span class="s">}</span>  <span class="c"># true if $a contains &quot;foo&quot;</span></li></ol></pre><p>The <code class="inline"><span class="q">//</span></code>
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 matching operator is documented in <a href="perlop.html">perlop</a>.  It operates on
<code class="inline"><span class="i">$_</span></code>
 by default, or can be bound to another variable using the <code class="inline">=~</code>

binding operator (also documented in <a href="perlop.html">perlop</a>).</p>
</li>
<li><a name="Simple-substitution"></a><b>Simple substitution</b>
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<pre class="verbatim"><ol><li> <span class="q">s/foo/bar/</span><span class="sc">;</span>               <span class="c"># replaces foo with bar in $_</span></li><li> <span class="i">$a</span> =~ <span class="q">s/foo/bar/</span><span class="sc">;</span>         <span class="c"># replaces foo with bar in $a</span></li><li> <span class="i">$a</span> =~ <span class="q">s/foo/bar/g</span><span class="sc">;</span>        <span class="c"># replaces ALL INSTANCES of foo with bar</span></li><li>                           <span class="c"># in $a</span></li></ol></pre><p>The <code class="inline"><a class="l_k" href="functions/s.html">s///</a></code> substitution operator is documented in <a href="perlop.html">perlop</a>.</p>
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</li>
<li><a name="More-complex-regular-expressions"></a><b>More complex regular expressions</b>
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<p>You don't just have to match on fixed strings.  In fact, you can match
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on just about anything you could dream of by using more complex regular
expressions.  These are documented at great length in <a href="perlre.html">perlre</a>, but for
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the meantime, here's a quick cheat sheet:</p>
677
<pre class="verbatim"><ol><li> .                   <span class="w">a</span> <span class="w">single</span> <span class="w">character</span></li><li> \<span class="q">s                  a whitespace character</span> <span class="s">(</span><span class="w">space</span><span class="cm">,</span> <span class="w">tab</span><span class="cm">,</span> <span class="w">newline</span><span class="cm">,</span></li><li>                     ...<span class="s">)</span></li><li> \<span class="w">S</span>                  <span class="w">non</span>-<span class="w">whitespace</span> <span class="w">character</span></li><li> \<span class="w">d</span>                  <span class="w">a</span> <span class="w">digit</span> <span class="s">(</span><span class="n">0</span>-<span class="n">9</span><span class="s">)</span></li><li> \<span class="w">D</span>                  <span class="w">a</span> <span class="w">non</span>-<span class="w">digit</span></li><li> \<span class="w">w</span>                  <span class="w">a</span> <span class="w">word</span> <span class="w">character</span> <span class="s">(</span><span class="w">a</span>-z<span class="cm">,</span> <span class="w">A</span>-<span class="w">Z</span><span class="cm">,</span> <span class="n">0</span>-<span class="n">9</span><span class="cm">,</span> <span class="w">_</span><span class="s">)</span></li><li> \<span class="w">W</span>                  <span class="w">a</span> <span class="w">non</span>-<span class="w">word</span> <span class="w">character</span></li><li> <span class="s">[</span><span class="w">aeiou</span><span class="s">]</span>             <span class="w">matches</span> <span class="w">a</span> <span class="w">single</span> <span class="w">character</span> <span class="w">in</span> <span class="w">the</span> <a class="l_k" href="functions/given.html">given</a> <span class="w">set</span></li><li> <span class="s">[</span>^<span class="w">aeiou</span><span class="s">]</span>            <span class="w">matches</span> <span class="w">a</span> <span class="w">single</span> <span class="w">character</span> <span class="w">outside</span> <span class="w">the</span> <a class="l_k" href="functions/given.html">given</a></li><li>                     <span class="i">set</span></li><li> <span class="s">(</span><span class="w">foo</span>|<span class="w">bar</span>|<span class="w">baz</span><span class="s">)</span>       <span class="w">matches</span> <span class="w">any</span> <span class="w">of</span> <span class="w">the</span> <span class="w">alternatives</span> <span class="w">specified</span></li><li></li><li> ^                   <span class="w">start</span> <span class="w">of</span> <span class="w">string</span></li><li> <span class="i">$                   end</span> <span class="w">of</span> <span class="w">string</span></li></ol></pre><p>Quantifiers can be used to specify how many of the previous thing you
678
want to match on, where "thing" means either a literal character, one
679
of the metacharacters listed above, or a group of characters or
680
metacharacters in parentheses.</p>
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<pre class="verbatim"><ol><li> <span class="i">*                   zero</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">more</span> <span class="w">of</span> <span class="w">the</span> <span class="w">previous</span> <span class="w">thing</span></li><li> +                   <span class="w">one</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">more</span> <span class="w">of</span> <span class="w">the</span> <span class="w">previous</span> <span class="w">thing</span></li><li> ?                   <span class="w">zero</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">one</span> <span class="w">of</span> <span class="w">the</span> <span class="w">previous</span> <span class="w">thing</span></li><li> <span class="s">{</span><span class="n">3</span><span class="s">}</span>                 <span class="w">matches</span> <span class="w">exactly</span> <span class="n">3</span> <span class="w">of</span> <span class="w">the</span> <span class="w">previous</span> <span class="w">thing</span></li><li> <span class="s">{</span><span class="n">3</span><span class="cm">,</span><span class="n">6</span><span class="s">}</span>               <span class="w">matches</span> <span class="w">between</span> <span class="n">3</span> <a class="l_k" href="functions/and.html">and</a> <span class="n">6</span> <span class="w">of</span> <span class="w">the</span> <span class="w">previous</span> <span class="w">thing</span></li><li> <span class="s">{</span><span class="n">3</span><span class="cm">,</span><span class="s">}</span>                <span class="w">matches</span> <span class="n">3</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">more</span> <span class="w">of</span> <span class="w">the</span> <span class="w">previous</span> <span class="w">thing</span></li></ol></pre><p>Some brief examples:</p>
<pre class="verbatim"><ol><li> <span class="q">/^\d+/</span>              <span class="w">string</span> <span class="w">starts</span> <span class="w">with</span> <span class="w">one</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">more</span> <span class="w">digits</span></li><li> <span class="q">/^$/</span>                <span class="w">nothing</span> <span class="w">in</span> <span class="w">the</span> <span class="w">string</span> <span class="s">(</span><span class="w">start</span> <a class="l_k" href="functions/and.html">and</a> <span class="w">end</span> <span class="w">are</span></li><li>                     <span class="w">adjacent</span><span class="s">)</span></li><li> /<span class="s">(</span>\<span class="w">d</span>\<span class="q">s){3}/         three digits, each followed by a whitespace</span></li><li>                     <span class="q">                     character (eg &quot;3 4 5 &quot;)</span></li><li> <span class="q"> /(a.)</span>+<span class="q">/             matches a string in which every odd-numbered</span></li><li>                     <span class="q">                     letter is a (eg &quot;abacadaf&quot;)</span></li><li></li><li> <span class="q"> # This loop reads from STDIN, and prints non-blank lines:</span></li><li> <span class="q"> while (&lt;&gt;) {</span></li><li>     <span class="q">     next if /</span>^<span class="i">$/</span><span class="sc">;</span></li><li>     <a class="l_k" href="functions/print.html">print</a><span class="sc">;</span></li><li> <span class="s">}</span></li></ol></pre></li>
683
<li><a name="Parentheses-for-capturing"></a><b>Parentheses for capturing</b>
684
<p>As well as grouping, parentheses serve a second purpose.  They can be
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used to capture the results of parts of the regexp match for later use.
The results end up in <code class="inline"><span class="i">$1</span></code>
, <code class="inline"><span class="i">$2</span></code>
 and so on.</p>
689
<pre class="verbatim"><ol><li> <span class="c"># a cheap and nasty way to break an email address up into parts</span></li><li></li><li> <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="i">$email</span> =~ <span class="q">/([^@]+)@(.+)/</span><span class="s">)</span> <span class="s">{</span></li><li>     <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Username is $1\n&quot;</span><span class="sc">;</span></li><li>     <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Hostname is $2\n&quot;</span><span class="sc">;</span></li><li> <span class="s">}</span></li></ol></pre></li>
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<li><a name="Other-regexp-features"></a><b>Other regexp features</b>
<p>Perl regexps also support backreferences, lookaheads, and all kinds of
other complex details.  Read all about them in <a href="perlrequick.html">perlrequick</a>,
<a href="perlretut.html">perlretut</a>, and <a href="perlre.html">perlre</a>.</p>
</li>
</ul>
<a name="Writing-subroutines"></a><h2>Writing subroutines</h2>
<p>Writing subroutines is easy:</p>
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<pre class="verbatim"><ol><li><a name="logger"></a> sub <span class="m">logger</span> <span class="s">{</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">$logmessage</span> = <a class="l_k" href="functions/shift.html">shift</a><span class="sc">;</span></li><li>    <a class="l_k" href="functions/open.html">open</a> <a class="l_k" href="functions/my.html">my</a> <span class="i">$logfile</span><span class="cm">,</span> <span class="q">&quot;&gt;&gt;&quot;</span><span class="cm">,</span> <span class="q">&quot;my.log&quot;</span> <a class="l_k" href="functions/or.html">or</a> <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;Could not open my.log: $!&quot;</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="i">$logfile</span> <span class="i">$logmessage</span><span class="sc">;</span></li><li> <span class="s">}</span></li></ol></pre><p>Now we can use the subroutine just as any other built-in function:</p>
<pre class="verbatim"><ol><li> <span class="i">logger</span><span class="s">(</span><span class="q">&quot;We have a logger subroutine!&quot;</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>What's that <code class="inline"><a class="l_k" href="functions/shift.html">shift</a></code>?  Well, the arguments to a subroutine are available
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to us as a special array called <code class="inline"><span class="i">@_</span></code>
 (see <a href="perlvar.html">perlvar</a> for more on that).
The default argument to the <code class="inline"><a class="l_k" href="functions/shift.html">shift</a></code> function just happens to be <code class="inline"><span class="i">@_</span></code>
.
So <code class="inline"><a class="l_k" href="functions/my.html">my</a> <span class="i">$logmessage</span> = <a class="l_k" href="functions/shift.html">shift</a><span class="sc">;</span></code>
 shifts the first item off the list of
arguments and assigns it to <code class="inline"><span class="i">$logmessage</span></code>
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.</p>
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<p>We can manipulate <code class="inline"><span class="i">@_</span></code>
 in other ways too:</p>
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<pre class="verbatim"><ol><li> <a class="l_k" href="functions/my.html">my</a> <span class="s">(</span><span class="i">$logmessage</span><span class="cm">,</span> <span class="i">$priority</span><span class="s">)</span> = <span class="i">@_</span><span class="sc">;</span>       <span class="c"># common</span></li><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">$logmessage</span> = <span class="i">$_</span>[<span class="n">0</span>]<span class="sc">;</span>                 <span class="c"># uncommon, and ugly</span></li></ol></pre><p>Subroutines can also return values:</p>
<pre class="verbatim"><ol><li><a name="square"></a> sub <span class="m">square</span> <span class="s">{</span></li><li>     <a class="l_k" href="functions/my.html">my</a> <span class="i">$num</span> = <a class="l_k" href="functions/shift.html">shift</a><span class="sc">;</span></li><li>     <a class="l_k" href="functions/my.html">my</a> <span class="i">$result</span> = <span class="i">$num</span> * <span class="i">$num</span><span class="sc">;</span></li><li>     <a class="l_k" href="functions/return.html">return</a> <span class="i">$result</span><span class="sc">;</span></li><li> <span class="s">}</span></li></ol></pre><p>Then use it like:</p>
<pre class="verbatim"><ol><li> <span class="i">$sq</span> = <span class="i">square</span><span class="s">(</span><span class="n">8</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>For more information on writing subroutines, see <a href="perlsub.html">perlsub</a>.</p>
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<a name="OO-Perl"></a><h2>OO Perl</h2>
<p>OO Perl is relatively simple and is implemented using references which
715
know what sort of object they are based on Perl's concept of packages.
716
However, OO Perl is largely beyond the scope of this document.
717
Read <a href="perlootut.html">perlootut</a> and <a href="perlobj.html">perlobj</a>.</p>
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<p>As a beginning Perl programmer, your most common use of OO Perl will be
in using third-party modules, which are documented below.</p>
<a name="Using-Perl-modules"></a><h2>Using Perl modules</h2>
<p>Perl modules provide a range of features to help you avoid reinventing
the wheel, and can be downloaded from CPAN ( <a href="http://www.cpan.org/">http://www.cpan.org/</a> ).  A
number of popular modules are included with the Perl distribution
itself.</p>
<p>Categories of modules range from text manipulation to network protocols
to database integration to graphics.  A categorized list of modules is
also available from CPAN.</p>
<p>To learn how to install modules you download from CPAN, read
729
<a href="perlmodinstall.html">perlmodinstall</a>.</p>
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<p>To learn how to use a particular module, use <code class="inline">perldoc <i>Module::Name</i></code>.
Typically you will want to <code class="inline">use <i>Module::Name</i></code>, which will then give
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you access to exported functions or an OO interface to the module.</p>
<p><a href="perlfaq.html">perlfaq</a> contains questions and answers related to many common
tasks, and often provides suggestions for good CPAN modules to use.</p>
<p><a href="perlmod.html">perlmod</a> describes Perl modules in general.  <a href="perlmodlib.html">perlmodlib</a> lists the
modules which came with your Perl installation.</p>
<p>If you feel the urge to write Perl modules, <a href="perlnewmod.html">perlnewmod</a> will give you
good advice.</p>
<a name="AUTHOR"></a><h1>AUTHOR</h1>
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<p>Kirrily "Skud" Robert &lt;skud@cpan.org&gt;</p>
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      <ul><li><a href="#NAME">NAME</a><li><a href="#DESCRIPTION">DESCRIPTION</a><ul><li><a href="#What-is-Perl%3f">What is Perl?</a><li><a href="#Running-Perl-programs">Running Perl programs</a><li><a href="#Safety-net">Safety net</a><li><a href="#Basic-syntax-overview">Basic syntax overview</a><li><a href="#Perl-variable-types">Perl variable types</a><li><a href="#Variable-scoping">Variable scoping</a><li><a href="#Conditional-and-looping-constructs">Conditional and looping constructs</a><li><a href="#Builtin-operators-and-functions">Builtin operators and functions</a><li><a href="#Files-and-I%2fO">Files and I/O</a><li><a href="#Regular-expressions">Regular expressions</a><li><a href="#Writing-subroutines">Writing subroutines</a><li><a href="#OO-Perl">OO Perl</a><li><a href="#Using-Perl-modules">Using Perl modules</a></ul><li><a href="#AUTHOR">AUTHOR</a></ul>
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