credit card validation

Validating a credit card number (the check sum bit, I mean. The having sufficient credit part you need to figure out yourself.) is a really simple process. You can practically do it in your head.

• Reverse the digits.
• Double the digits in the even positions (2, 4, 6, etc.).
• Add up all of the individual digits.
• If the sum mod 10 is zero, then the number is valid.

For example:

• 4111 1111 1111 1111
• 1111 1111 1111 1114
• 1212 1212 1212 1218
• 1 + 2 + 1 + 2 + 1 + 2 + 1 + 2 + 1 + 2 + 1 + 2 + 1 + 2 + 1 + 8 = 30
• 30 mod 10 = 0

Here's a little perl subroutine to do it for ya. Returns 1 for valid and 0 for invalid.

    sub is_valid_cc {
        my $cc = shift() || return 0;
     
        my @digits = reverse split( '', $cc );       # reverse the digits
        for ($i=0; $i < scalar(@digits); $i++) {                          
            $digits[$i] *= ($i%2) ? 2 : 1;           # double the evens
        }
        my $string = join( '', @digits );            # recombine into a string

        my $sum = 0;
        foreach ( split( '', $string ) ) {           # add the digits
            $sum += $_;
        }
         
        return ($sum % 10) ? 0 : 1;
    }