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.

  1. double the value of every other digit, counting backward from the second-last
  2. 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)
  3. add the leftover numbers together
  4. 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

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In reply to Luhn Number Golf by Cody Pendant

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