FA-95974 / Postal address normalization / Member archive

Alternating letter-digit postal code: forbidden letters are checked only at the first position · case 04

"K1U 4B1" is accepted although U is forbidden everywhere.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Strip and uppercase. A 7-character input with a space at index 3 loses the space. The result must be 6 characters alternating ASCII letter, digit, letter, digit, letter, digit. Letters D F I O Q U are forbidden everywhere; W and Z are forbidden in the first position only. Return "L9L 9L9" or None.

Why this case matters

Six-character alternating codes are typed with and without the space; rejecting or accepting the wrong letters silently drops or misroutes mail.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
forbidden letter in third position	`"K4U 7B4"`	`null`	Failed

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