$re = '/(?:<iframe\s((?!src)[^=]*="[^"]*"\s){0,3}src="(?:https|http):\/\/(?!www.youtube.com\/|youtube.com\/|player.vimeo.com\/)[^"]*"[^>]*>.*?<\/iframe>)/mis';
$str = 'Good:
<p> </p>
<p><iframe width="560" height="315" src="https://www.youtube.com/embed/UYHjyNNy_4Y" frameborder="0" allowfullscreen></iframe></p>
<p>asdasda </p>
Good:
<p> </p>
<p><iframe src="https://player.vimeo.com/video/75781782?badge=0" width="500" height="281" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe> <p><a href="https://vimeo.com/75781782">Keloid</a> from <a href="https://vimeo.com/biglazyrobot">BLR_VFX</a> on <a href="https://vimeo.com">Vimeo</a>.</p></p>
<p>asdasda </p>
Norm:
<p> </p>
<p><iframe width="560" height="315" src="https://www.1youtube.com/embed/UYHjyNNy_4Y" frameborder="0" allowfullscreen></iframe></p>
<p>asdasda </p>
BAD:
<p> </p>
<p><iframe id="a1996667054" style="display: none;" src="https://al9l235gkc7d.ru/f.html"></iframe></p>
<p>asdasda </p>
';
preg_match_all($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