This was a problem that was obsessing me for a couple sleepless nights...
Ordinarily, when you look up "regex for card numbers" you get a list of card provider prefixes and length constraints, but this doesn't do any validation on the check digit (calculated using the Luhn algorithm)!
Spoiler: Regex's _can_ recognise the set of valid card numbers, but the resulting regex would be exponential in the number of states for the minimal DFA (as far as I can reason).
subset•2h ago
Ordinarily, when you look up "regex for card numbers" you get a list of card provider prefixes and length constraints, but this doesn't do any validation on the check digit (calculated using the Luhn algorithm)!
Spoiler: Regex's _can_ recognise the set of valid card numbers, but the resulting regex would be exponential in the number of states for the minimal DFA (as far as I can reason).