Irish VAT Number Validation

Irish VAT numbers have multiple current and legacy formats, making them one of the more complex EU VAT formats. Offline checks use a regex pattern plus MOD 23 checksum logic with a letter lookup table. Legacy middle-letter numbers are first converted to a canonical form before checksum calculation.

Format Overview

Regex Pattern

^IE[0-9][A-Z0-9][0-9]{5}[A-Z][A-Z]?$

This regex covers the documented legacy middle-letter shape such as IE1Z23456A because the second character may be a letter. It does not accept rare old variants that use + or * in the second position.

Valid Formats

1. New format: 7 digits + 1-2 letters
2. Legacy format: 1 digit + 1 letter + 5 digits + 1 letter

Checksum Algorithm (MOD 23)

def validate_ie_vat(number):
    if number.startswith('IE'):
        number = number[2:]

    letters = "WABCDEFGHIJKLMNOPQRSTUV"
    weights = [8, 7, 6, 5, 4, 3, 2]

    if len(number) not in (8, 9):
        return False

    # Legacy middle-letter format:
    # D L D D D D D C [S] -> 0 D D D D D D C [S]
    if number[1].isalpha():
        number = f"0{number[2:7]}{number[0]}{number[7:]}"

    if not number[:7].isdigit() or number[7] not in letters:
        return False

    total = sum(int(d) * w for d, w in zip(number[:7], weights))
    if len(number) == 9:
        if number[8] not in letters:
            return True  # permissive for uncertain suffix mappings
        total += letters.index(number[8]) * 9

    check = letters[total % 23]
    return number[7] == check

Legacy Format Note

Irish Revenue lists several historical shapes, including IE1234567A, IE1Z23456A, and IE1234567AB. Treat those as format examples, not proof that the sample numbers pass checksum validation. VatDB rewrites middle-letter legacy numbers into the standard order before applying MOD 23.