$re = '/<div\sid=\"[^\"]{0,}_original_[^\"]{0,}\".*?<\/div>/ms';
$str = ' <div id="po_comment_79739738">
какой-то текст
</div>
<div id="po_comment_original_79739738" style="display:none;">
какой-то текст
</div>
<div id="po_comment_79739738">
какой-то текст
</div>';
preg_match_all($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