/(youtu(?:\.be|be\.com)\/(?:.*v(?:\/|=)|(?:.*\/)?)([\w'-]+))/i

Tested examples:
http://www.youtube.com/user/Scobleizer#p/u/1/1p3vcRhsYGo
http://www.youtube.com/watch?v=cKZDdG9FTKY&feature=channel
http://www.youtube.com/watch?v=yZ-K7nCVnBI&playnext_from=TL&videos=osPknwzXEas&feature=sub
http://www.youtube.com/ytscreeningroom?v=NRHVzbJVx8I
http://www.youtube.com/user/SilkRoadTheatre#p/a/u/2/6dwqZw0j_jY
http://youtu.be/6dwqZw0j_jY
http://www.youtube.com/watch?v=6dwqZw0j_jY&feature=youtu.be
http://youtu.be/afa-5HQHiAs
http://www.youtube.com/user/Scobleizer#p/u/1/1p3vcRhsYGo?rel=0
http://www.youtube.com/watch?v=cKZDdG9FTKY&feature=channel
http://www.youtube.com/watch?v=yZ-K7nCVnBI&playnext_from=TL&videos=osPknwzXEas&feature=sub
http://www.youtube.com/ytscreeningroom?v=NRHVzbJVx8I
http://www.youtube.com/embed/nas1rJpm7wY?rel=0
http://www.youtube.com/watch?v=peFZbP64dsU