ecial">,</span> <span class="identifier">what</span><span class="special">,</span> <span class="identifier">r</span><span class="special">))</span>
   <span class="special">{</span>
      <span class="comment">// extract $1 as a std::string:</span>
      <span class="keyword">return</span> <span class="identifier">what</span><span class="special">.</span><span class="identifier">str</span><span class="special">(</span><span class="number">1</span><span class="special">);</span>
   <span class="special">}</s