>the spec should be better than "a 14 digit number"

Actually even that would be wrong.

The spec for credit cards includes 13- to 16-digit numbers as well.

The spec would be, roughly, "a 13-to-16-digit Luhn number beginning with one of a list of prefixes."

There's an article about it here and a Perl implementation for checking here
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“Every bit of code is either naturally related to the problem at hand, or else it's an accidental side effect of the fact that you happened to solve the problem using a digital computer.” M-J D