ifier">logic_error</span></code> if the match_results object
        was not initialized.
      </p>
<p>
        <span class="bold"><strong>Effects</strong></span>: Returns sub-expression <span class="emphasis"><em>sub</em></span>
        as a string: <code class="computeroutput"><span class="identifier">string_type</span><span class="special">((*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">sub</span><span class="special">])</span></code>.
      </p>
<p>
  