Username specs

[James MacNeil]

Does anyone have a regular expression for validating the syntax of a username and password?  (If not, does anyone have the rules for valid names?)   I've noticed that the Perl client will let an invalid username/password proceed through the registration process, but will come back with a 465 error when your server sees it.  I'd like to tell the user that they've got a bad username before they go through the trouble of entering all their address info, etc.   Thanks