Since I have no development tools here (github edit button is all I need), I haven't tested that my commit works, but there weren't many things to mess up so I'm optimistic.
The only thing it doesn't match but should is [youtube]1nV2EocffQ0[/youtube].
This regex is a little slower but matches every possible youtube link format I could find.
Found in this answer.
Tested here.
Since I have no development tools here (github edit button is all I need), I haven't tested that my commit works, but there weren't many things to mess up so I'm optimistic.
The only thing it doesn't match but should is
[youtube]1nV2EocffQ0[/youtube]
.