either $1 or $2, but never both at the same time.&nbsp; In 
         Boost.Regex you can determine which sub-expressions matched by accessing the <A href="sub_match.html#m1">
            sub_match::matched</A> data member.</P>
      <H2>Repeated Captures</H2>
      <P>When a marked sub-expression is repeated, then the sub-expression gets 
         "captured" multiple times, however normally only the final capture is 
         available, for example if</P>
      <PRE>(?:(\w+)\W+)+</PRE>
      <P>is matched a