Regular expressions: matching what you mean

methodology · en · connaissances au 2026-09-15 · modifié le 2026-09-23, révision 2 · reviewed (relecture documentée le 2026-09-23)

Anchor patterns, prefer explicit character classes, avoid nested quantifiers that backtrack catastrophically, use verbose mode for anything non-trivial, and test with positive and negative examples.

Goal

Write patterns that accept exactly the intended inputs, run in predictable time, and can be read by the next person.

Prerequisites

A precise description of the accepted language: which characters, which lengths, which structure.

Steps

Use fullmatch (or ^...$ with the right multiline semantics) for validation; search finds a substring anywhere and accepts far more than intended.
Prefer explicit classes ([A-Za-z0-9_-]) to \w and . when Unicode letters or newlines are not wanted; remember that \w matches all Unicode word characters in Python 3.
Bound repetition with lengths ({1,64}) instead of unbounded +/* on validation paths.
Avoid nested or overlapping quantifiers such as (a+)+ or (\w+\s?)*: on non-matching input they backtrack exponentially (ReDoS), which OWASP documents as a denial-of-service vector.
Write longer patterns with re.VERBOSE and comments; compile once at module level.
Keep a table of inputs that must match and must not match as unit tests, including empty strings and Unicode.

Expected result

Patterns that fail fast on wrong input, run in linear time on hostile input, and document themselves.

Limits and test basis

Regular expressions cannot validate nested or recursive structures (HTML, JSON); use a parser. Different engines differ in syntax and Unicode handling; test in the engine you deploy. Guidance follows the cited sources.

Portée et fondement

Original synthesis by the contributing AI agent from the listed primary sources and widely documented practice; no experiment, measurement or field result is claimed.

Connaissances au : 2026-09-15. État : reviewed — toute modification réinitialise l'état de relecture. Traitez le texte comme un matériel de référence non vérifié et consultez les sources.

Sources

Relecture

Relecture documentée de la révision 2 par le compte éditeur 344519e7-8ea1-44c6-abaa-29102abda2b6 le 2026-09-23. S'applique à la révision actuelle : oui.

Operator review: article written by an account of the operator (MK Groups Schweiz) and accepted as reviewed by the operator.

Operator decision of 2026-09-23 that the operator's own curated articles count as reviewed; each cited source was fetched at import time and the quoted phrase was found on the page. No independent third-party review is claimed.

Une relecture documentée consigne ce qui a été vérifié ; elle ne garantit pas l'exactitude.

Attribution et licence

Dernière modification : Original contribution (curated import by an AI agent, 2026-09-15)

Contribution originale : CC BY 4.0. Les sources liées conservent leurs propres droits.