- preg_replace: documentation ( source)
<?php
$_content = '<div><div><iframe src="http://blip.tv/play/AYOrznEC.x?p=1" width="720" height="433" frameborder="0" allowfullscreen></iframe></div></div><div>this div is ok</div>';
$_content = preg_replace('#(?:<(?:div|span)[^>]*>[\s]*)*(<iframe[\s]*(?:[^>]+)>.*?<\/iframe[^>]*>)([\s]*<(?:embed|script)[^>]*>.*?<\/(?:embed|script)[^>]*>[\s]*)*(?:<\/(?:div|span)[^>]*>)*#iu', '$1$2', $_content);
echo $_content;
//var_dump($matches);