FA-72368 / Check-digit algorithms / Member archive

ISIN letters expand from one instead of ten · case 03

ISINs containing letters get the wrong computed check digit.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

Input a 12-character ISIN: two uppercase ASCII letters (country), nine uppercase alphanumerics, one check digit; anything else is "malformed". Letters expand to two digits (A=10 .. Z=35), then Luhn doubling starts at the rightmost digit of the expanded 11-character payload. Return [computed check digit, whether it matches].

Why this case matters

Securities settlement systems reject ISINs whose check digit does not match before routing trades.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression ["XS0K4JDSXBN1"]	`[2, false]`	`[4, false]`	Failed

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