$re = '~<!-- OPTIONAL -->(.*?)<!-- OPTIONAL END -->~s';
$str = '<p>something</p>
<!-- OPTIONAL -->
<p class="sdf"> some text</p>
<p> some other text</p>
<!-- OPTIONAL END -->
<p>The end</p>
<p>something</p>
<!-- OPTIONAL -->
<p class="sdf"> some text</p>
<p> some other text</p>
<!-- OPTIONAL END -->
<p>The end</p>';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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