tring that was matched by sub-expression
        <span class="emphasis"><em>n</em></span>. For example the expression:
      </p>
<pre class="programlisting">^(a*)[^a]*\1$</pre>
<p>
        Will match the string:
      </p>
<pre class="programlisting"><span class="identifier">aaabbaaa</span>
</pre>
<p>
        But not the string:
      </p>
<pre class="programlisting"><span class="identifier">aaabba</span>
</pre>
<div class="caution"><table border="0" summary="Caution">
<tr>
<td rowspan="2" align="center" v