Weird prime tests

What's your strangest way to check if a number is prime? Here's a Python one using regex:

import re def is_prime(n): return not re.match(r"^.?$|^(..+?)\1+$","\0"*n)

I find it hard to believe that a regex could identify primeness. My recollection is that encryption keys are usually not exhaustively proven but only proven “likely prime”