Restrict Usernames Help

[djxcee]

I'm trying to restrict usernames to the following characters only:

a-Z, 0-9, -, _ (dash and underscore) but can't seem to do so.

What should I add to Username Regular Expression to make what I want possible?

[Dismounted]

^[a-z0-9-_]+$
vBulletin automagically adds "#" at the start and "#siU" at the end (this is for those wondering why I didn't put it there).

[djxcee]

Thank you Dismounted!
Working great now :).