tor to be applied to.
      </p>
<h5>
<a name="boost_regex.syntax.basic_extended.h6"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.back_references"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.back_references">Back
        references:</a>
      </h5>
<p>
        An escape character followed by a digit <span class="emphasis"><em>n</em></span>, where <span class="emphasis"><em>n</em></span>
        is in the range 1-9, matches the same s