I was just looking at the about.com Perl site, and it has this article about finding if a number is a valid Luhn number, which, apparently, tells you it's a valid credit card number.
- double the value of every other digit, counting backward from the second-last
- add all the digits of the resulting numbers together (not the numbers, but the digits of the numbers, so if you've got 16, you add it as one plus six)
- add the leftover numbers together
- add them to the sum of the other lot of numbers
If what you've got is divisible by ten, it's a valid credit card number.
Their solution is at http://perl.about.com/library/weekly/aa080600h.htm, and of course it's quite long, because it's a teaching exercise.
I got thinking about it and how to do it more compactly, in the spirit of "Regular Expressions -- is there anything they can't do?" and tried to make it as short as I could.
Here's an attempt, but I'd love to see you make it shorter:
$n = '4564123800603607';
# this is not my credit card number
($o = reverse($n)) =~ s/.(.)/($1*2)/ge;
# every other number in it, times 2
# (reversed because then it will work
# for numbers of different lengths, as
# some cards are 13, not 16).
($e = reverse($n)) =~ s/(.)./$1/ge;
# the other numbers, not times 2
$o =~ s/(.)/$x+=$1/ge;
$e =~ s/(.)/$y+=$1/ge;
# add the digits
printf("%sValid.",(($x+$y)%10 == 0?'':'Not '));
# print the result