It's just making a regex like \A[^\n]*(\w)[^\n]*\n(?:[^\n]*\1[^\n]*\n)+\z which just says, I captured $1 and it has to exist on every line. Higher # versions just capture more and assert the new digits, in order. It's really very simple. rx() could be improved if you pegged certain captures with specified characters. I'm searching the entire space so I'm slower.
What's this have to do with ATM PINs?
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