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  27 <a name="boost.xpressive.keep"></a><div class="titlepage"></div>
  28 <div class="refnamediv">
  29 <h2><span class="refentrytitle">Function template keep</span></h2>
  30 <p>boost::xpressive::keep &#8212; Create an independent sub-expression. </p>
  31 </div>
  32 <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
  33 <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;
  34
  35 </span>
  36 <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">keep</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>
  37 <div class="refsect1">
  38 <a name="idp630420784"></a><h2>Description</h2>
  39 <p>Turn off back-tracking for a sub-expression. Any branches or repeats within the sub-expression will match only one way, and no other alternatives are tried.</p>
  40 <div class="note"><table border="0" summary="Note">
  41 <tr>
  42 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../doc/src/images/note.png"></td>
  43 <th align="left">Note</th>
  44 </tr>
  45 <tr><td align="left" valign="top"><p>keep(expr) is equivalent to the perl (?&gt;...) extension.</p></td></tr>
  46 </table></div>
  47 <p>
  48
  49 </p>
  50 <div class="variablelist"><table border="0" class="variablelist compact">
  51 <colgroup>
  52 <col align="left" valign="top">
  53 <col>
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  56 <td><p><span class="term">Parameters:</span></p></td>
  57 <td><div class="variablelist"><table border="0" class="variablelist compact">
  58 <colgroup>
  59 <col align="left" valign="top">
  60 <col>
  61 </colgroup>
  62 <tbody><tr>
  63 <td><p><span class="term"><code class="computeroutput">expr</code></span></p></td>
  64 <td><p>The sub-expression to modify. </p></td>
  65 </tr></tbody>
  66 </table></div></td>
  67 </tr></tbody>
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  72 <td align="left"></td>
  73 <td align="right"><div class="copyright-footer">Copyright &#169; 2007 Eric Niebler<p>
  74         Distributed under the Boost Software License, Version 1.0. (See accompanying
  75         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>)
  76       </p>
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