NAME

verify_cc(1) - a Perl program to verify a credit card number

SYNOPSIS

verify_cc -{ v | m | a | n } cardnumber

% verify_cc -v 1123581321345589
invalid card number: 1123581321345589

$ret = system("verify_cc -v 1123581321345589") >> 8;
die "verify_cc returned an error: $ret\n" if $ret;

DESCRIPTION

The verify_cc program is an adaptation of the Credit Card Verifier from Matt's Script Archive (but don't go there, please). It takes two arguments, both required:

card type

A single character (v, m, a, n) preceded by a hyphen, standing respectively for Visa, Mastercard, American Express, and Discover.

card number

A 13, 15, or 16 digit card number (depending on card type).

Following is a (more or less) line by line description of the program.

Initialization

Lines 1-17 are just starting up the program. Line 1 is the path to the Perl interpreter, as well as the -s switch for allowing commandline arguments. Lines 3-6 declare the strict and vars pragmas, as well as the global variables to declare, and the catch() subroutine, along with its prototype. Lines 9-17 produce the usage output, if the program is called incorrectly.

 1: #!/usr/bin/perl -s
 2: 
 3: use strict;
 4: use vars qw( $VERSION $USAGE $v $m $a $n );
 5: 
 6: sub catch ($$);
 7: 
 8: $VERSION = '1.00';
 9: $USAGE = << "END";
10: usage: verify_cc -[ v | m | a | n ] cardnumber
11: 
12: options:
13:   -v	Visa
14:   -m	Mastercard
15:   -a	American Express
16:   -n	Novus
17: END

Argument Processing

Lines 19-25 process the arguments to verify_cc, and set up a couple other variables for use (if all goes well). The four variables, $v, $m, $a, and $n are the valid options to verify_cc, and ONE of them can be true, at most. Thus, line 21 uses the bitwise XOR on them, to ensure one of them is true, and the rest are false. The program exits with status 1 (explained below) if the card type is set incorrectly, or if there is not a second argument. Lines 22-25 work with the card number, being sure it only contains digits; the length of the number is also recorded.

19: my ($card,$len,@digits,$sum);
20: 
21: ($v ^ $m ^ $a ^ $n) or catch 1, $USAGE;
22: $card = shift or catch 1, $USAGE;
23: $card =~ tr/- //d;
24: $card =~ /^(\d+)$/ or catch 2, "invalid card number: $card\n";
25: $len = length $card;

Quality Control

Lines 27-42 make sure that the card is of valid length for its type, and that the checksum of the digits is divisible by 10. Line 34 is special in function: if the number of digits is odd, then, because of the method of checksumming, a 0 is prepended to the list of digits, so that the checksumming process needn't be modified for an credit card of odd length.

26: catch 3, "invalid card length: $len\n" unless
27: 	$v && ($len == 13 || $len == 16) ||
28: 	$m && $len == 16 ||
29: 	$a && $len == 15 ||
30: 	$n && $len == 16;
32: 
33: @digits = split //, $card;
34: unshift @digits, 0 if $len % 2;
35: 
36: for (0..$#digits){
37: 	my $d = $digits[$_];
38: 	$sum += ($d * (2 - ($_ % 2)) > 9) ? ($d * 2 - 9) : $d;
39: }
40: catch 2, "invalid card number: $card\n" if $sum % 10;
41: 
42: catch 0, "$card is perfectly fine.\n";

The catch() Subroutine

Lines 45-49, the last of the program, define the catch() subroutine, which was predeclared much earlier. It receives a number and a message; the message is printed, and the program exits with the return status given it.

45: sub catch ($$) {
46: 	my ($errno,$msg) = @_;
47: 	print $msg;
48: 	exit $errno;
49: }

EXIT STATUS

The following exit values are returned:

0

The card number and type passed the checksum perfectly.

1

The program was invoked incorrectly. The usage message is returned.

2

The card number contained illegal characters (digits, spaces, and hyphens are allowed).

3

The card was of illegal length for its type.

AUTHOR

Jeff Pinyan, japhy+perl@pobox.com, CPAN ID: PINYAN

SEE ALSO

http://www.pobox.com/~japhy/perl/crap/001-ccver/article.pod for the accompanying article.