# NAME

CheckDigits::M23_002 - compute check digits for Tax Identification Number (IE)

# SYNOPSIS

``````  use Algorithm::CheckDigits;

\$dni = CheckDigits('tin_ie');

if (\$dni->is_valid('8473625E')) {
# do something
}

\$cn = \$dni->complete('1234567.W');
# \$cn = '1234567TW'

\$cd = \$dni->checkdigit('1234577IA');
# \$cd = 'I'

\$bn = \$dni->basenumber('1234577WW');
# \$bn = '1234577.W'
``````

# DESCRIPTION

## ALGORITHM

The irish TIN (Tax Identification Number) or VAT Regstration Number consists of 7 digits, a letter in the range from 'A' - 'W' as checksum, and an optionally letter in the range from 'A' - 'I' or the letter 'W'.

1. In reverse order, each digit is multiplied by a weight started at 2. (i.e. the number left from the check digit is multiplied with 2, the next with 3 and so on).

2. If there is an optional letter following the checksum letter (position 9), this letter is mapped to a numeric value based on the following mapping: "A" = 1, "B" = 2, ... "H" = 8, "I" = 9. "W" or absence of this letter means a value of 0. This numeric value is multiplied with 9.

4. The check digit is the sum from step 3 modulo 23. This number is expressed as the corresponding letter from the alphabet where A-V correspond to 1-22 and W stands for check digit 0.

## METHODS

is_valid(\$number)

Returns true only if `\$number` complies with the rules given above and there is a valid check digit at position eight.

Returns false otherwise,

complete(\$number)

The check digit for `\$number` is computed and inserted at position eight of `\$number`.

Returns the complete number with check digit or '' if `\$number` does not consist solely of digits and an optional letter at position nine.

basenumber(\$number)

Returns the basenumber of `\$number` if `\$number` has a valid check digit. As a placeholder for the checksum a point ('.') is inserted at position eight when the checksum contains the optional letter at position nine.

Return '' otherwise.

checkdigit(\$number)

Returns the checkdigit of `\$number` if `\$number` has a valid check digit.

Return '' otherwise.

None by default.

# AUTHOR

Mathias Weidner, `<mamawe@cpan.org>`