align="top" width="25"><img alt="[Caution]" src="../../../../../../doc/src/images/caution.png"></td>
<th align="left">Caution</th>
</tr>
<tr><td align="left" valign="top"><p>
          The POSIX standard does not support back-references for "extended"
          regular expressions, this is a compatible extension to that standard.
        </p></td></tr>
</table></div>
<h5>
<a name="boost_regex.syntax.basic_extended.h7"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.alternation">