What the Matched URLs code seems to do is take both of these URLs and strip them down to just username which then gets used in the embedded HTML to create the BBCode. So basically it works correctly for the live stream but not the video.
What I want to do is modify the expression so that it will work only for URLs of the form http://www.twitch.tv/username
Then I want to create another BBCode Media Site which will work for URLs of the form http://www.twitch.tv/username/c/digits
Any help from @Jake Bunce or any other regex gurus would be greatly appreciated.