Question

We all know that 42 is the answer to everything, but it’s news to me that it is a valid credit card number.

Try entering ’42’ or ‘42176’ into this jQuery Validation testing page and click OK.

What’s going on? I thought this was supposed to be the de-facto validation library. Even Microsoft is using it now, but it thinks ’42’ and ‘42176’ are valid credit card numbers?! It’s not even doing a length check. Surely I’m not responsible for adding a length check too? It’s called ‘creditcard’ validator and not ‘luhn’ validator.

Edit: hitchhiker humor aside – how would I go about patching the validation plugin to check length. is that easy?

Answer 1

This is probably because this validator merely checks that the number supplied satisfies the LUHN-10 algorithm (which 42 satisfies since 4*2 + 2 = 10 which is 0 modulo 10).

A better validator should maybe check for a minimal number of digits.

I’m not sure this corresponds to the very latest code from jQuery, but I found the snippet associated with credit card validation:

    // http://docs.jquery.com/Plugins/Validation/Methods/creditcard
    // based on http://en.wikipedia.org/wiki/Luhn
    creditcard: function(value, element) {
        if ( this.optional(element) )
            return "dependency-mismatch";
        // accept only digits and dashes
        if (/[^0-9-]+/.test(value))
            return false;
        var nCheck = 0,
            nDigit = 0,
            bEven = false;

        value = value.replace(/\D/g, "");

        for (n = value.length - 1; n >= 0; n--) {
            var cDigit = value.charAt(n);
            var nDigit = parseInt(cDigit, 10);
            if (bEven) {
                if ((nDigit *= 2) > 9)
                    nDigit -= 9;
            }
            nCheck += nDigit;
            bEven = !bEven;
        }

        return (nCheck % 10) == 0;
    },

… and as you see this merely check that all characters are digits and that LUHN-10 is satisfied, without any attention to a minial length.