I think I get what you are trying to do. what you want is only alphabet or
number allowed in username. Here is the regular expression for it.
PHP Code:
$regexp = "^[A-Za-z0-9]+$"
As I mention in the first part.
"+" mean one or more. If you omit the
"+", the
regular expression mean only one character allowed in string. But if you
put
"+", it mean one or more alphabet or number characters allowed.
From siLenTz
Hope, it is what you want...