mingw/html/ext/re/re.html

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  13 <big><strong><span class="block">&nbsp;re - Perl pragma to alter regular expression behaviour</span></strong></big>
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  22         <li><a href="#name">NAME</a></li>
  23         <li><a href="#synopsis">SYNOPSIS</a></li>
  24         <li><a href="#description">DESCRIPTION</a></li>
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  29 <p>
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  31 <h1><a name="name">NAME</a></h1>
  32 <p>re - Perl pragma to alter regular expression behaviour</p>
  33 <p>
  34 </p>
  35 <hr />
  36 <h1><a name="synopsis">SYNOPSIS</a></h1>
  37 <pre>
  38     use re 'taint';
  39     ($x) = ($^X =~ /^(.*)$/s);     # $x is tainted here</pre>
  40 <pre>
  41     $pat = '(?{ $foo = 1 })';
  42     use re 'eval';
  43     /foo${pat}bar/;                # won't fail (when not under -T switch)</pre>
  44 <pre>
  45     {
  46         no re 'taint';             # the default
  47         ($x) = ($^X =~ /^(.*)$/s); # $x is not tainted here</pre>
  48 <pre>
  49         no re 'eval';              # the default
  50         /foo${pat}bar/;            # disallowed (with or without -T switch)
  51     }</pre>
  52 <pre>
  53     use re 'debug';                # NOT lexically scoped (as others are)
  54     /^(.*)$/s;                     # output debugging info during
  55                                    #     compile and run time</pre>
  56 <pre>
  57     use re 'debugcolor';           # same as 'debug', but with colored output
  58     ...</pre>
  59 <p>(We use $^X in these examples because it's tainted by default.)</p>
  60 <p>
  61 </p>
  62 <hr />
  63 <h1><a name="description">DESCRIPTION</a></h1>
  64 <p>When <code>use re 'taint'</code> is in effect, and a tainted string is the target
  65 of a regex, the regex memories (or values returned by the m// operator
  66 in list context) are tainted.  This feature is useful when regex operations
  67 on tainted data aren't meant to extract safe substrings, but to perform
  68 other transformations.</p>
  69 <p>When <code>use re 'eval'</code> is in effect, a regex is allowed to contain
  70 <code>(?{ ... })</code> zero-width assertions even if regular expression contains
  71 variable interpolation.  That is normally disallowed, since it is a
  72 potential security risk.  Note that this pragma is ignored when the regular
  73 expression is obtained from tainted data, i.e.  evaluation is always
  74 disallowed with tainted regular expressions.  See <a href="file://C|\msysgit\mingw\html/pod/perlre.html#___code___">(?{ code }) in the perlre manpage</a>.</p>
  75 <p>For the purpose of this pragma, interpolation of precompiled regular
  76 expressions (i.e., the result of <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_qr_"><code>qr//</code></a>) is <em>not</em> considered variable
  77 interpolation.  Thus:</p>
  78 <pre>
  79     /foo${pat}bar/</pre>
  80 <p><em>is</em> allowed if $pat is a precompiled regular expression, even
  81 if $pat contains <code>(?{ ... })</code> assertions.</p>
  82 <p>When <code>use re 'debug'</code> is in effect, perl emits debugging messages when
  83 compiling and using regular expressions.  The output is the same as that
  84 obtained by running a <code>-DDEBUGGING</code>-enabled perl interpreter with the
  85 <strong>-Dr</strong> switch. It may be quite voluminous depending on the complexity
  86 of the match.  Using <code>debugcolor</code> instead of <code>debug</code> enables a
  87 form of output that can be used to get a colorful display on terminals
  88 that understand termcap color sequences.  Set <code>$ENV{PERL_RE_TC}</code> to a
  89 comma-separated list of <code>termcap</code> properties to use for highlighting
  90 strings on/off, pre-point part on/off.
  91 See <a href="file://C|\msysgit\mingw\html/pod/perldebug.html#debugging_regular_expressions">Debugging regular expressions in the perldebug manpage</a> for additional info.</p>
  92 <p>The directive <code>use re 'debug'</code> is <em>not lexically scoped</em>, as the
  93 other directives are.  It has both compile-time and run-time effects.</p>
  94 <p>See <a href="file://C|\msysgit\mingw\html/pod/perlmodlib.html#pragmatic_modules">Pragmatic Modules in the perlmodlib manpage</a>.</p>
  95 <table border="0" width="100%" cellspacing="0" cellpadding="3">
  96 <tr><td class="block" style="background-color: #cccccc" valign="middle">
  97 <big><strong><span class="block">&nbsp;re - Perl pragma to alter regular expression behaviour</span></strong></big>
  98 </td></tr>
  99 </table>
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