Disable certain characters in usernames

[Abronsyth]

OK, so there's this chunk of code;

      if($username == "SYSTEM"){
         $this->seterror("Cannot use SYSTEM as username.");
         return FALSE;
      }

But I'm wondering if I could do something like this to make it so the only characters that can be entered are alphanumeric and spaces? So if a user tries something like; "user_name" then they'll get the error; "Sorry, you cannot use special characters in your username."

Some folks use this sort of code;

PHP Code:

if(preg_match('/^[a-zA-Z0-9 ]+$/', $username)) {
} 


Do y'all think I'd be able to incorporate that somehow..?