# FreeBSD Manual Pages

```Algorithm::LUHN(3)    User Contributed Perl Documentation   Algorithm::LUHN(3)

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

SYNOPSIS
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");

DESCRIPTION
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".

FUNCTION
is_valid	CHECKSUMMED_NUM
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
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, ...)

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 <https://en.wikipedia.org/wiki/Damm_algorithm>
(Damm, not Damn).

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

I have also written a review of LUHN modules
<http://neilb.org/reviews/luhn.html>, which covers them in more detail
than this section.

REPOSITORY
<https://github.com/neilb/Algorithm-LUHN>

AUTHOR
This module was written by Tim Ayers
(http://search.cpan.org/search?author=TAYERS).