How to denie registration when unwhitelisted char is used?

[computer22]

Hello Guys,

I would like to denie the registration when another character than
abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWX YZ0123456789_
in the username is used.

Is that possible?
If yes, how?

[kh99]

In the ACP go to vBulletin Options->User Registration Options->Username Regular Expression and put in an expression to match only the characters you want. I haven't tried it, but I think you'd want
^[a-zA-Z0-9_]+$

You didn't mention the space character, if you want to allow spaces you'd have to put that in too. (The help for that option has other examples).

[computer22]

Great help, thank you very much. Didn't know it was that easy!