In addition (for the purposes of the exact example under discussion in this thread), I would also use $fulladdress (or $address) as the first parameter for $regex(), and switching ! (start-of-identd) for ^ (start-of-string) and @ (end-of-identd) for $ (end-of-string). They are functionally the same here, delimiting the string you wish to check. Alternately, since you are requiring the client to be unidentd'd in this example, you could start with the ~ as your start-of-match character since it cannot appear anywhere else in $fulladdress or $address.
on @*:JOIN:#: if ($regex($fulladdress, /~[a-z](?:\d{2,})@/i)) ban -k # $nick 2 Auto-ban: Spam bot.
