I’ve found an excellent online tool, where you can give it a regex, and it will spit out the PHP syntax for use with preg_replace, preg_match, etc.
I’m forever getting ‘Unknown modifier’ errors when trying to insert regex’s into my PHP code. This sorts it out for you.
http://regex.larsolavtorvik.com/
Another one that I found recently:
http://www.myregextester.com/index.php