FA-72704 / Check-digit algorithms / Member archive

UK VAT reads only the last check digit · case 04

Most valid VAT numbers are rejected.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

UK VAT registration number check. Spaces are removed, the text is upper-cased and an optional GB prefix removed; the rest must be 9 ASCII digits (else "malformed"). Weights 8..2 apply to the first seven digits; the last two digits form a number c. Return "mod97" if (sum + c) is divisible by 97, "mod9755" if (sum + c + 55) is, else "invalid".

Why this case matters

Invoice processing screens supplier VAT numbers before reclaiming input tax.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression ["155142529"]	`"invalid"`	`"mod9755"`	Failed

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