It could match on c1, but I believe since most (all?) regex parsers parse left-to-right, it will match the a, look for another a-z character or a digit, find b,

It could match on c1, but I believe since most (all?) regex parsers parse left-to-right, it will match the a, look for another a-z character or a digit, find b, repeat, find c, then find 1 which completes the pattern.