p34eu

All amateur radio call signs:
[a-zA-Z0-9]{1,3}[0123456789][a-zA-Z0-9]{0,3}[a-zA-Z]

Non-US call signs:
\b(?!K)(?!k)(?!N)(?!n)(?!W)(?!w)(?!A[A-L])(?!a[a-l])[a-zA-Z0-9][a-zA-Z0-9]?[a-zA-Z0-9]?[0123456789][a-zA-Z0-9][a-zA-Z0-9]?[a-zA-Z0-9]?[a-zA-Z0-9]?\b

US call signs:
[AKNWaknw][a-zA-Z]{0,2}[0123456789][a-zA-Z]{1,3}