$re = '/<p.*?>.*?<iframe.*?><\/iframe><\/p>/is';
$str = '<p style="text-align: center; ">
<iframe height="360" src="http://example.com/videoembed/9338/" frameborder="0" width="640"></iframe></p>
<p style="text-align: center;"><iframe allowfullscreen="" frameborder="0" height="360" src="http://example.com/videoembed/9718/" width="640"></iframe></p>';
preg_match_all($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