</span>: Returns the length of sub-expression
        <span class="emphasis"><em>sub</em></span>, that is to say: <code class="computeroutput"><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">sub</span><span class="special">].</span><span class="identifier">length</span><span class="special">()</span></code>.
      </p>
<p>
        The overloads that accept a string refer to a named sub-expression <span class="emphasis"><em>n</em></span>