# Call this subroutine with the credit card number you wish to validate 
# (spaces and hyphens allowed).  It will return 1 if the number passes the
# checksum checks, 0 otherwise.  Note that this does NOT go to a credit bureau
# to authenticate the user's credit worthiness!

# See "Web Security, A Step-by-Step Reference Guide" by Lincoln Stein, page 46

sub check_cc {
    my $num = shift;
    my $sum = 0;
    my @digits = $num=~/(\d)/g;;
    my @weights = (1,2) x (@digits/2 + 1);
    shift @weights unless @digits % 2;
    foreach (@digits) {
	my $weighted = $_ * shift @weights;
	$weighted -= 9 if $weighted > 9;
	$sum += $weighted;
    }
    return ($sum % 10) == 0;
}