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<title>re - Perl pragma to alter regular expression behaviour</title>
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<h1><a>re - Perl pragma to alter regular expression behaviour</a></h1>
<p><a name="__index__"></a></p>
<!-- INDEX BEGIN -->
<ul>
<li><a href="#name">NAME</a></li>
<li><a href="#synopsis">SYNOPSIS</a></li>
<li><a href="#description">DESCRIPTION</a></li>
</ul>
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<h1><a name="name">NAME</a></h1>
<p>re - Perl pragma to alter regular expression behaviour</p>
<p>
</p>
<hr />
<h1><a name="synopsis">SYNOPSIS</a></h1>
<pre>
<span class="keyword">use</span> <span class="variable">re</span> <span class="string">'taint'</span><span class="operator">;</span>
<span class="operator">(</span><span class="variable">$x</span><span class="operator">)</span> <span class="operator">=</span> <span class="operator">(</span><span class="variable">$^X</span> <span class="operator">=~</span> <span class="regex">/^(.*)$/s</span><span class="operator">);</span> <span class="comment"># $x is tainted here</span>
</pre>
<pre>
<span class="variable">$pat</span> <span class="operator">=</span> <span class="string">'(?{ $foo = 1 })'</span><span class="operator">;</span>
<span class="keyword">use</span> <span class="variable">re</span> <span class="string">'eval'</span><span class="operator">;</span>
<span class="regex">/foo${pat}bar/</span><span class="operator">;</span> <span class="comment"># won't fail (when not under -T switch)</span>
</pre>
<pre>
<span class="operator">{</span>
<span class="keyword">no</span> <span class="variable">re</span> <span class="string">'taint'</span><span class="operator">;</span> <span class="comment"># the default</span>
<span class="operator">(</span><span class="variable">$x</span><span class="operator">)</span> <span class="operator">=</span> <span class="operator">(</span><span class="variable">$^X</span> <span class="operator">=~</span> <span class="regex">/^(.*)$/s</span><span class="operator">);</span> <span class="comment"># $x is not tainted here</span>
</pre>
<pre>
<span class="keyword">no</span> <span class="variable">re</span> <span class="string">'eval'</span><span class="operator">;</span> <span class="comment"># the default</span>
<span class="regex">/foo${pat}bar/</span><span class="operator">;</span> <span class="comment"># disallowed (with or without -T switch)</span>
<span class="operator">}</span>
</pre>
<pre>
<span class="keyword">use</span> <span class="variable">re</span> <span class="string">'debug'</span><span class="operator">;</span> <span class="comment"># NOT lexically scoped (as others are)</span>
<span class="regex">/^(.*)$/s</span><span class="operator">;</span> <span class="comment"># output debugging info during</span>
<span class="comment"># compile and run time</span>
</pre>
<pre>
<span class="keyword">use</span> <span class="variable">re</span> <span class="string">'debugcolor'</span><span class="operator">;</span> <span class="comment"># same as 'debug', but with colored output</span>
<span class="operator">...</span>
</pre>
<p>(We use $^X in these examples because it's tainted by default.)</p>
<p>
</p>
<hr />
<h1><a name="description">DESCRIPTION</a></h1>
<p>When <code>use re 'taint'</code> is in effect, and a tainted string is the target
of a regex, the regex memories (or values returned by the m// operator
in list context) are tainted. This feature is useful when regex operations
on tainted data aren't meant to extract safe substrings, but to perform
other transformations.</p>
<p>When <code>use re 'eval'</code> is in effect, a regex is allowed to contain
<code>(?{ ... })</code> zero-width assertions even if regular expression contains
variable interpolation. That is normally disallowed, since it is a
potential security risk. Note that this pragma is ignored when the regular
expression is obtained from tainted data, i.e. evaluation is always
disallowed with tainted regular expressions. See <a href="../lib/Pod/perlre.html#___code___">(?{ code }) in the perlre manpage</a>.</p>
<p>For the purpose of this pragma, interpolation of precompiled regular
expressions (i.e., the result of <a href="../lib/Pod/perlfunc.html#item_qr_"><code>qr//</code></a>) is <em>not</em> considered variable
interpolation. Thus:</p>
<pre>
<span class="regex">/foo${pat}bar/</span>
</pre>
<p><em>is</em> allowed if $pat is a precompiled regular expression, even
if $pat contains <code>(?{ ... })</code> assertions.</p>
<p>When <code>use re 'debug'</code> is in effect, perl emits debugging messages when
compiling and using regular expressions. The output is the same as that
obtained by running a <code>-DDEBUGGING</code>-enabled perl interpreter with the
<strong>-Dr</strong> switch. It may be quite voluminous depending on the complexity
of the match. Using <code>debugcolor</code> instead of <code>debug</code> enables a
form of output that can be used to get a colorful display on terminals
that understand termcap color sequences. Set <code>$ENV{PERL_RE_TC}</code> to a
comma-separated list of <code>termcap</code> properties to use for highlighting
strings on/off, pre-point part on/off.
See <a href="../lib/Pod/perldebug.html#debugging_regular_expressions">Debugging regular expressions in the perldebug manpage</a> for additional info.</p>
<p>The directive <code>use re 'debug'</code> is <em>not lexically scoped</em>, as the
other directives are. It has both compile-time and run-time effects.</p>
<p>See <a href="../lib/Pod/perlmodlib.html#pragmatic_modules">Pragmatic Modules in the perlmodlib manpage</a>.</p>
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