preg_match() Unknown modifier '[' help
Try the following:
<?php
$str = "http://www.youtube.com/ytscreeningroom?v=NRHVzbJVx8I";
$pattern = '#(?<=v=)[a-zA-Z0-9-]+(?=&)|(?<=[0-9]/)[^&\n]+|(?<=v=)[^&\n]+#';
preg_match($pattern, $str, $matches);
print_r($matches);
?>
Note, I'm using # as a delimiter here simply because the regular expression above contains forward slashes and escaping them makes the expression more difficult to read. This cleans it up by just a few pixels.