Regex Tester

How to use Regex Tester

1. Type a pattern into Pattern, without the surrounding slashes, and tick the Flags you need (g is on by default). Try an example loads a date pattern with named groups.
2. Paste text into Test text, drop a file on it or use Open file. Matches are highlighted in Matches as you type.
3. Check the match table for each match’s index, text and capture groups, including named groups.
4. Enter a Replacement such as $1,
   lt;name>,
   amp; or $ to preview the replaced text, then copy it or use Send to….
5. Read Pattern explained for a one-line description of each part of the pattern.

How it works

The pattern is compiled with the browser’s own RegExp, so the syntax and behaviour are exactly those of JavaScript in your browser, including named groups, lookbehind and, with the u flag, \p{…} Unicode properties. Matches are found with RegExp.prototype.exec, and the replacement preview uses String.prototype.replace, so it replaces every match with g and only the first without it.

Matching runs in a Web Worker, shortly after you stop typing. If it doesn’t finish within 1 second, the worker is terminated, which is the only way to stop a pattern that backtracks catastrophically, and a fresh one is used for the next try. Empty matches advance by one character (one code point with u), so patterns such as a* can’t loop forever.

Pattern explained is a best-effort tokenizer, not a full parser: it describes escapes, classes, groups, quantifiers, anchors and back-references one by one.

Limits

• JavaScript regex syntax only. PCRE, Python or .NET features such as possessive quantifiers, atomic groups, inline (?i) flags or \A / \Z aren’t supported unless your browser’s RegExp accepts them.
• Flags g, i, m, s, u, y and d are available; the v (unicodeSets) flag isn’t offered.
• Matching stops after 1 second and shows a backtracking warning instead of results.
• Up to 5,000 matches are found; the table lists the first 500. The test text can be up to 1,000,000 characters.
• Empty (zero-length) matches appear in the table but can’t be highlighted in the text.

Privacy

The pattern and text are processed in your browser and never uploaded or stored. The Share button (Copy share link) puts the pattern, flags, test text and replacement in the link’s # fragment, which browsers don’t send to servers; links are limited in size, so long texts show Too large to share. A pattern sent here with Send to…, or a result you send on, passes through this tab’s session storage and is removed as soon as the receiving tool reads it.

Frequently asked questions

Why does my pattern only find one match?

Without the g (global) flag, a JavaScript regex stops after the first match. Tick g to find every match.

Do I need to escape slashes?

No. Type the pattern without the surrounding /…/ and flags; a / inside is matched literally. If you paste the pattern into JavaScript code as a literal, escape it there as \/.

What does “Stopped after 1 s” mean?

The pattern took too long, usually because of catastrophic backtracking: nested quantifiers like (a+)+ can take exponential time on text that almost matches. Make the inner part more specific or remove the nesting.

Will a pattern that works here work in Python or Java?

Not always. This tool uses your browser’s JavaScript engine. Common syntax is shared, but named groups, lookbehind, Unicode classes and replacement syntax ($1 here, \1 in Python) differ between languages.

How do I use a named group in the replacement?

Name it with (?<year>\d{4}) in the pattern, then write