pecial">[[.</span><span class="identifier">NUL</span><span class="special">.]]</span>
</pre>
<p>
        matches a NUL character.
      </p>
<h6>
<a name="boost_regex.syntax.basic_extended.h14"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.equivalence_classes"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.equivalence_classes">Equivalence
        classes:</a>
      </h6>
<p>
        An expression of the form <code class="computeroutput">