$re = '/<iframe .*?\bsrc="(https?://fast\.player\.liquidplatform\.com/pApiv2/embed/e50a2b66dc19adc532f288eb4bf2d302/[^"]+)/m';
$str = '<iframe src="https://fast.player.liquidplatform.com/pApiv2/embed/e50a2b66dc19adc532f288eb4bf2d302/f2c5f6ca3a4610c55d70cb211ef9d977" webkitallowfullscreen="" width="490">
<iframe allowfullscreen="" frameborder="0" height="276" mozallowfullscreen="" scrolling="no" src="https://fast.player.liquidplatform.com/pApiv2/embed/e50a2b66dc19adc532f288eb4bf2d302/%20f2c5f6ca3a4610c55d70cb211ef9d977" webkitallowfullscreen="" width="490"></iframe>';
preg_match_all($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