Online Regex Tester with Live Match Highlighting
Quick answer
An online regex tester that highlights every match as you type — with capture groups, flags and a replace preview — using JavaScript's real RegExp engine. Nothing to install, no account to create, and your test text never leaves the browser tab you are working in.
Last updated: October 16, 2026
Debugging a regex should not require running code. Open this page, and the loop is one keystroke long: change the pattern, see the highlights move. The test text stays where you put it — in your tab — because matching happens in your browser, not on a server that logs what you paste. Flags are toggle buttons, groups are listed per match, and the replace preview shows the substitution result before you trust it.
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Matching runs in your browser with JavaScript's own RegExp engine — nothing is sent anywhere. Pattern, flags and text auto-save locally.
Online Regex Tester runs entirely in your browser — nothing you paste or type is uploaded to any server, and no account is needed.
How do I debug a regular expression online?
- 1
Paste the problematic pattern between the slashes — the error message below it points at unbalanced groups or unescaped characters.
- 2
Paste real sample text in the editor; highlight updates live so you can see which lines the pattern catches and which it misses.
- 3
Add capture groups and check the match list underneath — each entry shows its position, its numbered groups and its named groups.
- 4
Use the Replaced tab to try a substitution ($1, $&, $<name>) and copy the final text when it is right.
Built for the debug loop
Instant feedback on every keystroke
There is no run button because there is nothing to run — matching is recomputed as you type, exactly like a live linter. The gap between idea and verification shrinks to zero.
Positions you can act on
Each match is listed with its index in the text, so "the pattern matched somewhere" becomes "it matched at character 214, group 2 is empty" — the detail you need to fix a data-extraction rule.
Flags as one-click toggles
g, i, m, s and u are buttons with tooltips, because forgetting the g flag is the most common regex bug there is. Watch the match count change the moment you toggle.
Engine-accurate results
The tool uses JavaScript's own RegExp — the same engine that will run your pattern in the browser or in Node. What you see highlighted is what production gets, including its error messages.
Nothing to install, anywhere
Works in every modern browser on desktop, tablet and phone — helpful when the pattern arrives while you are away from your IDE and you just need to check one string.
Auto-save for interrupted debugging
Pattern, flags, text and replacement persist in your browser's localStorage. Close the tab mid-debug; the setup is restored exactly as you left it — locally, on your machine.
Why does the same regex behave differently in different tools?
Regex is a family of dialects, not one language. JavaScript, PCRE (PHP), Python's re, Java and .NET share the core syntax but disagree at the edges — lookbehind support, how named groups are written, what the s and u flags mean, and how unicode property escapes (\p{L}) behave. A pattern verified in a PCRE-based tester can legitimately fail in JavaScript.
This tester runs the JavaScript flavor, which makes it the honest preview for browser validation and Node.js code. If your target is Python or PHP, use this page to get the structure right, then re-run the full pattern in that language — differences concentrate in the exotic constructs, and 95% of everyday patterns transfer unchanged.
Questions this tool answers
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Frequently asked questions
Can I use this regex tester without an account?
Yes — the tool works fully without any signup. Sharing features on other sites need accounts because they store your test data on servers; this tester keeps everything in your browser, so there is simply nothing to register for.
Does the tester support named capture groups?
Yes. Named groups (?<name>…) appear in the match list as name="value" next to the numbered groups, and you can reference them in the replace tab with $<name> — the same syntax JavaScript itself uses.
How many matches does it show?
It counts up to 2,000 matches and highlights the first 500 for rendering speed; the match list shows the first 100 with a note about the remainder. Real-world log scanning stays snappy, and the count tells you the true total.
Can I test multiline patterns?
Yes — the test editor is a full multiline editor. Combine real newlines with the m flag so ^ and $ anchor to each line, and with the s flag if you need . to cross line boundaries.
Is my test text saved or shared anywhere?
No server sees it — matching is computed locally. Your input auto-saves to localStorage in your own browser for convenience, and clearing the editor (or your browser data) removes it completely.
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