Which regex engine does the tester use?

It uses the browser's native JavaScript ECMAScript RegExp engine, so matching behavior corresponds to JavaScript and Node.js.

Can a pathological pattern freeze the page?

Matching runs in a worker with a timeout. A timeout stops the operation and reports a likely catastrophic-backtracking case.

What can I inspect in a match?

The result highlights matches in the test string and reports capture groups, named groups and positions.

Regex Tester

Test JavaScript regular expressions with live match highlighting, capture groups, substitution, and shareable links.

Regex Railroad Diagrams and NFA/DFA Steppers: Seeing the Machine Inside Your Pattern

Browsers run regular expressions as backtracking engines, not the finite automata textbooks describe. Railroad diagrams, Thompson's construction and a step debugger close that gap — and make ReDoS visible before it ships.

Log Parsing with Regex: Turn Lines into Rows

Turn raw Apache, Nginx, and syslog lines into structured data using named capture groups, and export the result as table, JSON, or CSV.

Find and Replace with Regular Expressions: A Practical Guide

How capture groups, alternation, and lazy vs. greedy matching turn search-and-replace from tedious editing into precise transformation.

Best Free Developer Tools in 2026: An Honest Comparison

The best free, browser-based developer tools compared — CyberChef, Regex101, JWT.io, and the privacy-first alternatives. What to use for JSON, regex, JWTs, hashing, UUIDs, and more.