title: How can I find position in string for submatch in boost::regex
body: <p>after <code>boost::regex_search(start, end, what, pattern)</code> I can find start position of full match in search string by calling <code>what.position()</code>. </p>
  
  <p>How can I find those positions for submatches?</p>
  
  <p>I need to have code like this:</p>
  
  <pre><code>if(boost::regex_search(start, end, what, pat))
  {
      int len = what["namedGroup"].length();
      int pos = what["namedGroup"].position();
  }
  </code></pre>