Regular expressions (regex) are powerful patterns used to search, match, and manipulate text. They're essential for form validation (email, phone, URL formats), log analysis (extracting patterns from server logs), data extraction (parsing structured text), and find-and-replace operations in code editors.

Common use cases include validating user input in web forms, searching codebases with precise patterns, parsing configuration files, and extracting specific data from large text dumps. This tester runs entirely client-side — your text and patterns never leave your device.