FA-72830 / Check-digit algorithms / Member archive

DNI validation is case-sensitive · case 05

Numbers typed in lowercase are reported as mismatches or malformed.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Spanish DNI/NIE control letter: input upper-cased, hyphens removed, then nine characters: eight digits (or X/Y/Z followed by seven digits, the prefix standing for 0/1/2) and a letter (else "malformed"). The letter is "TRWAGMYFPDXBNJZSQVHLCKE"[number mod 23]. Return [expected letter, match].

Why this case matters

Tax and KYC forms validate the control letter of Spanish national and foreigner identity numbers.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression ["12345678z"]	`["Z", false]`	`["Z", true]`	Failed

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