$re = '/<div[^>]*id="THIS"[^>]*>(.*?)(</div>)+?
/s';
$str = '<div class="viewport viewport_h" style = "overflow: hidden;" >
<div id="THIS" class="overview overview_h">
<ul>
<li>some txt to be captured</li>
<li>some txt to be captured</li>
<li>some txt to be captured</li>
</ul>
<div>
" some text to be captured"
</div>
</div>
</div>
"some text not to be captured"
</div>
<div class="scrollbar_h">
<div class="track_h"></div>';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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