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

This works by first converting the number to unary, i.e. 5 will be ‘11111’ and 3 will be ‘111’ and so on. First, it tries to match 0 or 1 in the LHS and then uses backreferences to try and match multiples of 2, 3, 4 and so on until a match is found or string length is exceeded.

For a deeper analysis please read: https://iluxonchik.github.io/regular-expression-check-if-number-is-prime/