I've simplified the regex slightly but you might want to do a few searches to see if there's any alternative URLs such as https:// or youtu.be links if so, the regex can probably be adjusted but this should cover the basic case for now and stop it from being so greedy:
Code:
/\[url=http:\/\/(?:www\.)*+youtube\.com\/watch\?v=([\w\-]+)\].+\[\/url\]/siU