#boost_regex.syntax.basic_extended.character_sets">Character
        sets:</a>
      </h5>
<p>
        A character set is a bracket-expression starting with <code class="literal">[</code>
        and ending with <code class="literal">]</code>, it defines a set of characters, and
        matches any single character that is a member of that set.
      </p>
<p>
        A bracket expression may contain any combination of the following:
      </p>
<h6>
<a name="boost_regex.syntax.basic_extended.h9"></a>
       