Regular expression

I want to use first letter of one tag (band) to format another tag and want to replace the first letter to # if not alphabet character using regular expression
$regexp($left(%band%,1),(?([A-Z])$0|#))
The result is blank. Any idea?
I've tried $regexp($left(%band%,1),[^A-Z],#) also but I get error :frowning:

Thank you,
Victor

$regexp($left(%band%,1),^\d,#)

Kind regards,
Florian