# nebx Regex Tester

> Free regex tester and debugger: live match highlighting, capture groups, plain-English explanations, substitution, unit tests and code generation. Runs in your browser; nothing is uploaded.

Part of nebx (https://nebx.ai/). About nebx and its other tools: https://nebx.ai/llms.txt

- Live match highlighting with capture groups and named groups
- Plain-English explanation of every part of the pattern
- All JavaScript flags: g, i, m, s, u, v, y, d
- Substitution preview with $1, $<name>, $& and more
- Unit tests: strings that must or must not match
- Code generation for JavaScript, Python, PHP, Go, Java and C#
- Quick reference and a library of common patterns
- Protection against catastrophic backtracking (runs in a worker with a time limit)
- Share link that keeps everything in the URL fragment

Q: Which regex flavor does it use?
A: JavaScript (ECMAScript), using your browser's own RegExp engine, so results match what your JavaScript code will do. Other flavors such as PCRE, Python, Java or .NET mostly share the same syntax but differ in details; the code generator warns about the common differences.

Q: Is my pattern or test text uploaded?
A: No. Matching runs in your browser and nothing is sent to a server. Your work is saved only in this browser. A share link keeps the pattern and text in the part of the URL after #, which browsers don't send to servers.

Q: What happens if my regex is very slow?
A: Matching runs in a background worker with a time limit. If a pattern takes too long, usually because of catastrophic backtracking such as (a+)+$, it is stopped and you get a warning instead of a frozen page.

Q: Can I test replacements?
A: Yes. Open Substitution and type a replacement such as $1, $<name>, $& or literal text to preview the result for the whole test string.

Q: Can I generate code from my regex?
A: Yes. The Code tab writes ready-to-paste snippets for JavaScript, Python, PHP, Go, Java and C#, with the pattern escaped for each language.