Username Registration (A-Z, 0-9 Characters Only?)

[Harlequin]

I'm not sure if this has already been done. I've searched through the threads here in an attempt to find it, but maybe I'm totally missing it. Surely someone out there has created a modification (or if they haven't, here's a suggestion? :P) that'd allow admins to control registration usernames to their board to accept only characters A-Z, 0-9..

No spaces, no special ascii-type letters, none of that sort of thing.

I'm just looking for an old-fashioned filter such as this.

Is this available? Would you point me in the right direction if it were? :P

If it's not available, I'd love for this to be created and/or be suggested in what to go about looking up to create this.