ble to a basic_string</A>, they can be <A href="sub_match.html#o21">compared 
            to a string</A>, <A href="sub_match.html#o81">added to a string</A>, or <A href="sub_match.html#oi">
            streamed out to an output stream</A>.</P>
      <H2>Unmatched Sub-Expressions</H2>
      <P>When a regular expression match is found there is no need for all of the marked 
         sub-expressions to have participated in the match, for example the expression:</P>
      <P>(abc)|(def)</P>
      <P>can match 