eregi and alfa numeric characters.

Hi all, I want a regular expression process such that, no characters except for the followings will not be allowed. 0-9 a-z A-Z ~ @ # $ % ( ) _ - + = [ ] { } < ? > I create a web site with membership. I have a problem. Some body uses a username GUEST and someother get's the same username (in fact not the same, but displays as a same username, because, second username is GUESTx where x replace with the code ALT+231 so I want to give a sequence of characters to the user, such that user cannot use except for this crahcters. I wrote all the characters above... best regards...