http://www.oembed.com/ <-- this is the way to go as far as i'm concerned going forward.. i already use it on my Wordpress portal... it 'just works', if you get the idea.
bbcode.inc.php should do what it currently does for 'Magic URLs' for sites that are oEmbed servers (there are many more than shown there).
then there is no custom BBcode or regexes involved at all.
additionally, if the server changes the embed, it's dynamically changed for any consumer (ubb.threads) software..
yes, custom bbcode tags are cool when i wrote em, but this would enhance it and prevent regex headaches i've seen all too often..
so combine the two and you can 'have it all'
and of course the admin should have the option to enable only what he/she wants too
