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  26 <a name="boost.xpressive.before"></a><div class="titlepage"></div>
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  28 <h2><span class="refentrytitle">Function template before</span></h2>
  29 <p>boost::xpressive::before &#8212; Look-ahead assertion. </p>
  30 </div>
  31 <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
  32 <div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../../xpressive/reference.html#header.boost.xpressive.regex_primitives_hpp" title="Header &lt;boost/xpressive/regex_primitives.hpp&gt;">boost/xpressive/regex_primitives.hpp</a>&gt;
  33
  34 </span>
  35 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <a class="link" href="../../Expr.html" title="Concept Expr">Expr</a><span class="special">&gt;</span> <span class="emphasis"><em><span class="identifier">unspecified</span></em></span> <span class="identifier">before</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> expr<span class="special">)</span><span class="special">;</span></pre></div>
  36 <div class="refsect1">
  37 <a name="id3918957"></a><h2>Description</h2>
  38 <p>before(expr) succeeds if the expr sub-expression would match at the current position in the sequence, but expr is not included in the match. For instance, before("foo") succeeds if we are before a "foo". Look-ahead assertions can be negated with the bit-compliment operator.</p>
  39 <div class="note"><table border="0" summary="Note">
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  41 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../doc/src/images/note.png"></td>
  42 <th align="left">Note</th>
  43 </tr>
  44 <tr><td align="left" valign="top"><p>before(expr) is equivalent to the perl (?=...) extension. ~before(expr) is a negative look-ahead assertion, equivalent to the perl (?!...) extension.</p></td></tr>
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  46 <p>
  47
  48 </p>
  49 <div class="variablelist"><table border="0">
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  51 <tbody><tr>
  52 <td><p><span class="term">Parameters:</span></p></td>
  53 <td><div class="variablelist"><table border="0">
  54 <col align="left" valign="top">
  55 <tbody><tr>
  56 <td><p><span class="term"><code class="computeroutput">expr</code></span></p></td>
  57 <td><p>The sub-expression to put in the look-ahead assertion. </p></td>
  58 </tr></tbody>
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  65 <td align="left"></td>
  66 <td align="right"><div class="copyright-footer">Copyright &#169; 2007 Eric Niebler<p>
  67         Distributed under the Boost Software License, Version 1.0. (See accompanying
  68         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
  69       </p>
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