Regular expressions, or regexes, are compact pattern languages used to search, validate, extract, and replace text. They can match simple strings, but many regex engines also support grouping, repetition, lookarounds, and other features that make them surprisingly expressive. In some implementations, overly complex patterns can even behave like small programs.
In cyber security, regexes are everywhere: input validation, log analysis, malware detection, URL filtering, and intrusion signatures. That power makes them useful for defense, but also risky when patterns are slow, ambiguous, or hard to review. Poorly written regexes can cause denial of service through excessive backtracking, and attackers may exploit weak validation rules by crafting inputs that bypass a pattern. Security teams therefore treat regexes as code: they test them, limit their scope, and prefer clear patterns over clever ones.