Regex Tester

Advanced online regex tester with real-time matching, accurate capture groups, a plain-English pattern explainer, catastrophic-backtracking (ReDoS) protection, replace-with-diff, and code export for 7 languages. Runs in a Web Worker so a pathological pattern can never freeze your browser.

Regex Tester — Build, Test, and Debug Regular Expressions

Regular expressions are the most powerful text-matching tool in any programmer's kit, but they're notoriously hard to write correctly. Our Regex Tester gives you a live playground — type a pattern, paste your test text, and see matches highlighted in real time. It goes further than a basic tester: a plain-English explainer breaks your pattern down token by token, captured groups (including named groups) are listed with their exact values and positions, a replace mode shows a colour-coded diff, and you can export the finished pattern as working code in seven languages.

Just as important, matching runs inside a background Web Worker with a hard time limit. If you write a pattern that backtracks catastrophically — the kind that hangs most online testers and can even take down a production server (a "ReDoS") — it is detected up front and, if it still runs long, terminated safely without ever freezing your browser tab. Useful for log parsing, data validation, scraping, and form validation. No signup, no cost.

How to Test Regex

1. Enter Pattern

Type your regex pattern. Choose flags — case-insensitive (i), multiline (m), global (g), etc.

2. Paste Test Text

Paste any sample text — emails, log lines, code, CSV data, anything you want to match against.

3. View Live Matches

Every match is highlighted in the test text as you type, each in its own colour, with a live count and match time.

4. Inspect & Export

See each match's position and captured groups, read the plain-English explanation, or copy the pattern as code for your language.

All testing happens in your browser. Your patterns and test data stay private.

Regex Tester Features

Live Highlighting

Matches highlight as you type. Adjust the pattern and see results immediately.

All Flags

Toggle g (global), i (case-insensitive), m (multiline), s (dot-all), u (unicode), y (sticky).

Catastrophic-Backtracking Protection

Risky patterns (ReDoS) are flagged before you run them, and matching runs in a background worker with a time limit — so a pathological pattern can never freeze the tab, unlike most online testers.

Plain-English Explainer

Every token in your pattern is described in words — anchors, classes, quantifiers, groups and lookarounds — so you can learn and debug at a glance.

Accurate Capture Groups

Every group is listed with its value and character positions. Named groups are labelled by name — correctly, even when two groups capture the same text.

Replace with Diff

Test replacements with $1, $2, $<name> and $& — and see exactly what was removed and added in a colour-coded preview.

Code Export (7 languages)

Copy your pattern as ready-to-run JavaScript, Python, PHP, Java, Go, C# or Ruby, with flags mapped correctly for each.

All Flags & a Cheatsheet

Toggle g, i, m, s, u and y, load ready-made patterns from the library, and keep the quick-reference cheatsheet a click away.

Free and Private

All processing happens in your browser. No signup, no tracking, no limits.

Regex Tester — FAQs

JavaScript (ECMAScript) regex flavour. It's similar to Python and PCRE but has subtle differences. Lookbehinds, named groups, and Unicode property escapes are all supported in modern browsers.

Mostly yes — the core syntax is similar across languages. But verify any advanced patterns (e.g. recursion, possessive quantifiers, conditional patterns) in the target language since support varies.

Common causes: unescaped special characters (use \. for literal dot, \\ for literal backslash), unclosed parentheses or brackets, or incorrect quantifier usage. The tool shows the exact error your browser's regex engine reports, so you can fix it immediately.

Some patterns — classically a repeated group that itself repeats, like (a+)+$ — can force the engine to try an exponential number of paths on certain input. This is called a ReDoS (Regular-expression Denial of Service) and it can hang a browser tab or a server for seconds, minutes, or longer. Most online testers run matching on the main thread and simply freeze. This tool analyses your pattern and warns you when it spots that shape, and it runs the actual matching in a background Web Worker with a hard time limit — if a pattern runs too long it is terminated and reported, and the page stays fully responsive throughout.

Yes. Open the Code Snippets panel and pick from JavaScript, Python, PHP, Java, Go, C# or Ruby. The pattern and flags are translated into idiomatic, ready-to-run code for each language — for example the JavaScript g flag is dropped for Python's re.finditer, and i/m/s are mapped to each language's own option names.

Yes, completely free. No signup, no limits, and everything runs locally in your browser.

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