$re = '/(?<=<a href=\")([^"]*)-([^&"]*)(?=\.html\">)/m';
$str = '<div id="breadcrumb-section">
<ol id="breadcrumbs">
<li class="first">
<span>
<a href="index.html">Technologies</a>
</span>
</li>
<li>
<span>
<a href="Some-Technologies-Documentation_218464400.html">Some Technologies Documentation</a>
</span>
</li>
<li>
<span>
<a href="Some-Other-Documentation_268370090.html">Some Other Documentation</a>
</span>
</li>
<li>
<span>
<a href="Another-Documentation_268370112.html">Another Documentation</a>
</span>
</li>
</ol>
</div>';
$subst = "$1_$2";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php