FA-72758 / Check-digit algorithms / Member archive

Amex prefix test uses a range · case 03

Diners-style 35 and 36 prefixes with 15 digits are routed to amex.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

Card number screening: spaces removed, nonempty ASCII digits required (else "malformed"). Network by prefix and length: visa 4 with 13/16/19 digits; amex 34 or 37 with 15; mastercard 51-55 or 2221-2720 with 16; discover 6011, 644-649 or 65 with 16-19; otherwise "unknown". Return [network, Luhn validity].

Why this case matters

Checkout forms route card numbers to the right processor and reject Luhn failures before authorization.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression ["351552971521401"]	`["amex", true]`	`["unknown", true]`	Failed

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