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<html><head><title>use re (Programming Perl)</title><!-- STYLESHEET --><link rel="stylesheet" type="text/css" href="../style/style1.css"><!-- METADATA --><!--Dublin Core Metadata--><meta name="DC.Creator" content=""><meta name="DC.Date" content=""><meta name="DC.Format" content="text/xml" scheme="MIME"><meta name="DC.Generator" content="XSLT stylesheet, xt by James Clark"><meta name="DC.Identifier" content=""><meta name="DC.Language" content="en-US"><meta name="DC.Publisher" content="O'Reilly & Associates, Inc."><meta name="DC.Source" content="" scheme="ISBN"><meta name="DC.Subject.Keyword" content=""><meta name="DC.Title" content="use re"><meta name="DC.Type" content="Text.Monograph"></head><body><!-- START OF BODY --><!-- TOP BANNER --><img src="gifs/smbanner.gif" usemap="#banner-map" border="0" alt="Book Home"><map name="banner-map"><AREA SHAPE="RECT" COORDS="0,0,466,71" HREF="index.htm" ALT="Programming Perl"><AREA SHAPE="RECT" COORDS="467,0,514,18" HREF="jobjects/fsearch.htm" ALT="Search this book"></map><!-- TOP NAV BAR --><div class="navbar"><table width="515" border="0"><tr><td align="left" valign="top" width="172"><a href="ch31_16.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0"></a></td><td align="center" valign="top" width="171"><a href="ch31_01.htm">Chapter 31: Pragmatic Modules</a></td><td align="right" valign="top" width="172"><a href="ch31_18.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td></tr></table></div><hr width="515" align="left"><!-- SECTION BODY --><h2 class="sect1">31.17. use re</h2><p>This pragma controls the use of regular expressions. It has four possible invocations: "<tt class="literal">taint</tt>" and "<tt class="literal">eval</tt>",which are lexically scoped, plus "<tt class="literal">debug</tt>" and "<tt class="literal">debugcolor</tt>",which aren't.<blockquote><pre class="programlisting">use re 'taint'; # Contents of $match are tainted if $dirty was also tainted.($match) = ($dirty =~ /^(.*)$/s); # Allow code interpolation:use re 'eval';$pat = '(?{ $var = 1 })'; # embedded code execution/alpha${pat}omega/; # won't fail unless under -T # and $pat is tainteduse re 'debug'; # like "perl -Dr"/^(.*)$/s; # output debugging info during # compile time and run timeuse re 'debugcolor'; # same as 'debug', # but with colored output</pre></blockquote>When <tt class="literal">use re 'taint'</tt> is in effect and a tainted string is thetarget of a regex, the numbered regex variables and values returnedby the <tt class="literal">m//</tt> operator in list context are all tainted. This isuseful when regex operations on tainted data aren't meant to extractsafe substrings, but to perform other transformations. See the discussionon tainting in <a href="ch23_01.htm">Chapter 23, "Security"</a>.</p><p>When <tt class="literal">use re 'eval'</tt> is in effect, a regex is allowedto contain assertions that execute Perl code, which are of the form<tt class="literal">(?{ ... })</tt>, even when the regex containsinterpolated variables. Execution of code segments resulting fromvariable interpolation into a regex is normally disallowed forsecurity reasons: you don't want programs that read patterns fromconfig files, command-line arguments, or CGI form fields to suddenlystart executing arbitrary code if they weren't designed to expect thispossibility. This use of the pragma allows only untainted strings tobe interpolated; tainted data will still cause an exception to beraised (if you're running with taint checks enabled). See also <a href="ch05_01.htm">Chapter 5, "Pattern Matching"</a>, and <a href="ch23_01.htm">Chapter 23, "Security"</a>.</p><p>For the purposes of this pragma, interpolation of precompiled regularexpressions (produced by the <tt class="literal">qr//</tt> operator) is not consideredvariable interpolation. Nevertheless, when you build the<tt class="literal">qr//</tt> pattern it needs to have <tt class="literal">use re 'eval'</tt> in effectif any of its interpolated strings contain code assertions. For example:<blockquote><pre class="programlisting">$code = '(?{ $n++ })'; # code assertion$str = '\b\w+\b' . $code; # build string to interpolate$line =~ /$str/; # this needs use re 'eval'$pat = qr/$str/; # this also needs use re 'eval'$line =~ /$pat/; # but this doesn't need use re 'eval'</pre></blockquote>Under <tt class="literal">use re 'debug'</tt>, Perl emits debugging messages when compilingand when executing regular expressions. The output is the same as thatobtained by running a "debugging Perl" (one compiled with<tt class="userinput"><b>-DDEBUGGING</b></tt> passed to the C compiler) and then executing your Perlprogram under Perl's <tt class="userinput"><b>-Dr</b></tt> command-line switch. Depending on howcomplicated your pattern is, the resulting output can be overwhelming.Calling <tt class="literal">use re 'debugcolor'</tt> enables more colorful output that can beuseful, provided your terminal understands color sequences. Set your<tt class="literal">PERL_RE_TC</tt> environment variable to a comma-separated list of relevant<em class="emphasis">termcap</em>(5) properties for highlighting. For more details, see <a href="ch20_01.htm">Chapter 20, "The Perl Debugger"</a>.</p><!-- BOTTOM NAV BAR --><hr width="515" align="left"><div class="navbar"><table width="515" border="0"><tr><td align="left" valign="top" width="172"><a href="ch31_16.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0"></a></td><td align="center" valign="top" width="171"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0"></a></td><td align="right" valign="top" width="172"><a href="ch31_18.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0"></a></td></tr><tr><td align="left" valign="top" width="172">31.16. use overload</td><td align="center" valign="top" width="171"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0"></a></td><td align="right" valign="top" width="172">31.18. use sigtrap</td></tr></table></div><hr width="515" align="left"><!-- LIBRARY NAV BAR --><img src="../gifs/smnavbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links"><p><font size="-1"><a href="copyrght.htm">Copyright © 2001</a> O'Reilly & Associates. 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