d for sub-expression i.
      </p>
<p>
        <span class="bold"><strong>Returns</strong></span>: <code class="computeroutput"><span class="special">(*</span><span class="keyword">this</span><span class="special">)[</span><span class="identifier">i</span><span class="special">].</span><span class="identifier">captures</span><span class="special">();</span></code>
      </p>
<p>
        <span class="bold"><strong>Preconditions</strong></span>: the library must be built
        and used with BOOST_REGEX_MATC