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<h2><span class="refentrytitle">Function template check</span></h2>
<p>boost::xpressive::check — For adding user-defined assertions to your regular expressions. </p>
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<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="../../xpressive/reference.html#header.boost.xpressive.regex_actions_hpp" title="Header <boost/xpressive/regex_actions.hpp>">boost/xpressive/regex_actions.hpp</a>>
</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">></span> <span class="emphasis"><em><span class="identifier">unspecified</span></em></span> <span class="identifier">check</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&</span> t<span class="special">)</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="idm45555020019248"></a><h2>Description</h2>
<p>
A <a class="link" href="../../xpressive/user_s_guide.html#boost_xpressive.user_s_guide.semantic_actions_and_user_defined_assertions.user_defined_assertions">user-defined assertion</a> is a kind of semantic action that evaluates a Boolean lambda and, if it evaluates to false, causes the match to fail at that location in the string. This will cause backtracking, so the match may ultimately succeed.</p>
<p>To use <code class="computeroutput">check()</code> to specify a user-defined assertion in a regex, use the following syntax:</p>
<pre class="programlisting"><span class="identifier">sregex</span> <span class="identifier">s</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">_d</span> <span class="special">>></span> <span class="identifier">_d</span><span class="special">)</span><span class="special">[</span><span class="identifier">check</span><span class="special">(</span> <span class="identifier">XXX</span> <span class="special">)</span><span class="special">]</span><span class="special">;</span> <span class="comment">// XXX is a custom assertion</span>
</pre>
<p> The assertion is evaluated with a <code class="computeroutput"><code class="computeroutput"><a class="link" href="sub_match.html" title="Struct template sub_match">sub_match</a></code><></code> object that delineates what part of the string matched the sub-expression to which the assertion was attached.</p>
<p><code class="computeroutput">check()</code> can be used with an ordinary predicate that takes a <code class="computeroutput"><code class="computeroutput"><a class="link" href="sub_match.html" title="Struct template sub_match">sub_match</a></code><></code> object as follows:</p>
<pre class="programlisting"><span class="comment">// A predicate that is true IFF a sub-match is</span>
<span class="comment">// either 3 or 6 characters long.</span>
<span class="keyword">struct</span> <span class="identifier">three_or_six</span>
<span class="special">{</span>
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">(</span><span class="special">)</span><span class="special">(</span><span class="identifier">ssub_match</span> <span class="keyword">const</span> <span class="special">&</span><span class="identifier">sub</span><span class="special">)</span> <span class="keyword">const</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">sub</span><span class="special">.</span><span class="identifier">length</span><span class="special">(</span><span class="special">)</span> <span class="special">==</span> <span class="number">3</span> <span class="special">||</span> <span class="identifier">sub</span><span class="special">.</span><span class="identifier">length</span><span class="special">(</span><span class="special">)</span> <span class="special">==</span> <span class="number">6</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span><span class="special">;</span>
<span class="comment">// match words of 3 characters or 6 characters.</span>
<span class="identifier">sregex</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">bow</span> <span class="special">>></span> <span class="special">+</span><span class="identifier">_w</span> <span class="special">>></span> <span class="identifier">eow</span><span class="special">)</span><span class="special">[</span> <span class="identifier">check</span><span class="special">(</span><span class="identifier">three_or_six</span><span class="special">(</span><span class="special">)</span><span class="special">)</span> <span class="special">]</span> <span class="special">;</span>
</pre>
<p> Alternately, <code class="computeroutput">check()</code> can be used to define inline custom assertions with the same syntax as is used to define semantic actions. The following code is equivalent to above:</p>
<pre class="programlisting"><span class="comment">// match words of 3 characters or 6 characters.</span>
<span class="identifier">sregex</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">bow</span> <span class="special">>></span> <span class="special">+</span><span class="identifier">_w</span> <span class="special">>></span> <span class="identifier">eow</span><span class="special">)</span><span class="special">[</span> <span class="identifier">check</span><span class="special">(</span><span class="identifier">length</span><span class="special">(</span><span class="identifier">_</span><span class="special">)</span><span class="special">==</span><span class="number">3</span> <span class="special">||</span> <span class="identifier">length</span><span class="special">(</span><span class="identifier">_</span><span class="special">)</span><span class="special">==</span><span class="number">6</span><span class="special">)</span> <span class="special">]</span> <span class="special">;</span>
</pre>
<p> Within a custom assertion, <code class="computeroutput">_</code> is a placeholder for the <code class="computeroutput"><code class="computeroutput"><a class="link" href="sub_match.html" title="Struct template sub_match">sub_match</a></code><></code> That delineates the part of the string matched by the sub-expression to which the custom assertion was attached. </p>
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<td><p><span class="term">Parameters:</span></p></td>
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<td><p><span class="term"><code class="computeroutput">t</code></span></p></td>
<td><p>The UnaryPredicate object or Boolean semantic action.</p></td>
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<td align="right"><div class="copyright-footer">Copyright © 2007 Eric Niebler<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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>)
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