ChecksumValidationOverRegex
title: Checksum Validation over Regex type: technique created: 2026-09-20 last_updated: 2026-09-20 related: [] sources: ["https://abstractnonsense.xyz/blog/2025-08-31-can-a-regex-match-valid-card-numbers/"] radar_quadrant: Techniques radar_ring: Assess radar_position: outer
Teams that need to find or scrub payment card numbers in text often reach for a single regular expression. Checksum Validation over Regex is the practice of confirming a candidate number with the card checksum itself, in ordinary code, rather than trying to encode the checksum inside a pattern.
A blog analysis shows that the Luhn checksum used by card numbers can be recognised by a finite automaton, which means an equivalent regular expression must exist. In practice the author generated such expressions and found them to be roughly 32.4 million and 48.2 million characters long for even-length and odd-length numbers. Python's re module could run them, but common command-line tools ran out of memory.
The small automaton, or a plain checksum function, is far more compact than the equivalent pattern. A practical pipeline uses a simple pattern to find candidate digit runs and then validates each with the checksum. The author also warns against relying on regular expressions alone for financial compliance and personal-data scrubbing.
The technique is placed in Assess at the outer position. It rests on a single blog post, its scope is narrow, and there is no first-person use. It is worth remembering when designing data-scrubbing steps, but it is not a broad practice that warrants a stronger placement.
