<code class="computeroutput">string_type</code> containing all the characters that compare equal disregrarding case to the one passed in. This function can only be called if <code class="computeroutput"><code class="computeroutput"><a class="link" href="has_fold_case.html" title="Struct template has_fold_case">has_fold_case</a></code>&lt;<code class="computeroutput"><a class="link" href="c_regex_traits.html" title="Struct template c_regex_traits">c_regex_traits</a></code>&lt;Char&gt; &gt;::value</code> is <