$dirtyCodePattern = "/(<\!\-\-\[if[^>]*>)/misU";
preg_match_all($dirtyCodePattern,$body,$rest,PREG_PATTERN_ORDER);
foreach($rest[0] as $i => $val)
$body = str_replace($rest[0][$i],"",$body);
Fonte: http://www.expressolivre.org/newbb/viewtopic.php?forum=5&topic_id=1329&post_id=9456