Algorithm::LUHN - Calculate the Modulus 10 Double Add Double checksum


  use Algorithm::LUHN qw/check_digit is_valid/;

  $c = check_digit("43881234567");
  print "It works\n" if is_valid("43881234567$c");

  $c = check_digit("A2C4E6G8"); # this will cause an error

  print "Valid LUHN characters are:\n";
  my %vc = Algorithm::LUHN::valid_chars();
  for (sort keys %vc) {
    print "$_ => $vc{$_}\n";

  Algorithm::LUHN::valid_chars(map {$_ => ord($_)-ord('A')+10} A..Z);
  $c = check_digit("A2C4E6G8");
  print "It worked again\n" if is_valid("A2C4E6G8$c");


This module calculates the Modulus 10 Double Add Double checksum, also known as the LUHN Formula. This algorithm is used to verify credit card numbers and Standard & Poor's security identifiers such as CUSIP's and CSIN's.

You can find plenty of information about the algorithm by searching the web for "modulus 10 double add double".



This function takes a credit-card number and returns true if the number passes the LUHN check.

Ie it returns true if the final character of CHECKSUMMED_NUM is the correct checksum for the rest of the number and false if not. Obviously the final character does not factor into the checksum calculation. False will also be returned if NUM contains in an invalid character as defined by valid_chars(). If NUM is not valid, $Algorithm::LUHN::ERROR will contain the reason.

This function is equivalent to

  substr $N,length($N)-1 eq check_digit(substr $N,0,length($N)-1)

For example, 4242 4242 4242 4242 is a valid Visa card number, that is provided for test purposes. The final digit is '2', which is the right check digit. If you change it to a '3', it's not a valid card number. Ie:

    is_valid('4242424242424242');   # true
    is_valid('4242424242424243');   # false
check_digit NUM

This function returns the checksum of the given number. If it cannot calculate the check_digit it will return undef and set $Algorithm::LUHN::ERROR to contain the reason why.

valid_chars LIST

By default this module only recognizes 0..9 as valid characters, but sometimes you want to consider other characters as valid, e.g. Standard & Poor's identifers may contain 0..9, A..Z, @, #, *. This function allows you to add additional characters to the accepted list.

LIST is a mapping of character => value. For example, Standard & Poor's maps A..Z to 10..35 so the LIST to add these valid characters would be (A, 10, B, 11, C, 12, ...)

Please note that this adds or re-maps characters, so any characters already considered valid but not in LIST will remain valid.

If you do not provide LIST, this function returns the current valid character map.


Algorithm::CheckDigits provides a front-end to a large collection of modules for working with check digits.

Business::CreditCard provides three functions for checking credit card numbers. Business::CreditCard::Object provides an OO interface to those functions.

Business::CardInfo provides a class for holding credit card details, and has a type constraint on the card number, to ensure it passes the LUHN check.

Business::CCCheck provides a number of functions for checking credit card numbers.

Regexp::Common supports combined LUHN and issuer checking against a card number.

Algorithm::Damm implements a different kind of check digit algorithm, the Damm algorithm (Damm, not Damn).

Math::CheckDigits implements yet another approach to check digits.

I have also written a review of LUHN modules, which covers them in more detail than this section.



This module was written by Tim Ayers (


Copyright (c) 2001 Tim Ayers. All rights reserved.


This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.