A regular expression, or regex, is a pattern language used to find, validate, extract, or replace text. Security tools and applications use regex to check usernames, file paths, branch names, log entries, and other input against expected formats. Because a regex can describe very precise text rules, it is useful for filtering and policy enforcement.
In cyber security, regex matters because it often sits in trust decisions. A bad pattern can let malicious input pass validation, while an overly complex pattern can cause heavy CPU use or trigger parser bugs. Attackers may abuse regex-based rules to bypass checks, crash services, or reach unsafe code paths if the pattern engine is flawed. Defenders use careful pattern design, input limits, and server-side review to reduce risk. In CI/CD platforms, regex may control pipeline behavior, so a defect in the regex compiler can become a serious attack surface.