If I'm understanding you correctly there's no need to use regular expressions for this.

Code:
var %address = :nick!nick@hhx-48524CF5.dyn.optonline.net

; for the nickname
$right($gettok(%address, 1, 33), -1)

; for the masked address
$mask(%address, 2)

Spelling mistakes, grammatical errors, and stupid comments are intentional.