Free Regex Tester & Debugger

The Regex Tester is a free online tool that lets you test and debug regular expressions in real-time with match highlighting and explanations.

Yes, it is completely free with no registration required. All pattern matching happens client-side in your browser.

Yes, the Regex Tester provides human-readable explanations of your regular expression patterns to help you understand and debug them.

Absolutely. The Regex Tester & Debugger processes everything client-side in your browser. No data is uploaded to or stored on any server. Your content remains private on your device at all times.

Yes, the Regex Tester & Debugger is fully responsive and works on smartphones and tablets. You can use it on any device with a modern web browser -- no app download required.

No account or registration is needed. Simply open the Regex Tester & Debugger in your browser and start using it immediately. There are no sign-up walls or usage restrictions.

Simply enter your input in the provided field, adjust any settings to your preference, and the tool will process it instantly. You can then copy the result to your clipboard or download it.

The Regex Tester & Debugger works in all modern browsers including Chrome, Firefox, Safari, Edge, and Opera. For the best experience, use the latest version of your preferred browser.

Flags change how the engine applies your pattern. The g (global) flag finds every match instead of stopping at the first. The i (ignore case) flag matches letters regardless of case. The m (multiline) flag makes the ^ and $ anchors attach to each line rather than only the whole string. The s (dotall) flag lets the dot match newline characters too. The u (unicode) flag turns on full Unicode handling and stricter parsing, which matters for emoji and non-Latin text. The y (sticky) flag matches only from the engine's current position, which is useful when tokenizing input piece by piece. Flags can be combined freely, so gi finds every match while ignoring case. This tool exposes all six as toggles, so you can flip them on and off and watch the highlighted matches and the match count update live to see exactly how each one affects your result.

Catastrophic backtracking happens when a pattern has nested or overlapping quantifiers, such as (.*)+ or (a+)+, that force the engine to try an exponential number of ways to match before giving up. On a short, well-formed string it looks fine, but on slightly longer or malformed input the matching time can explode from microseconds to seconds, effectively hanging your program. You avoid it by making quantifiers non-overlapping, using more specific character classes instead of broad ones like .*, and anchoring patterns where possible. This tool flags risky constructs as you type and shows an execution-time badge ranging from Fast to Very Slow, so you can spot a dangerous pattern before it reaches production. The step-by-step debugger, capped at 100 steps so a runaway pattern cannot lock up the page, also lets you watch how the engine reaches each match. Paste your expression above and watch the badge to confirm it stays fast.

A capture group, written with plain parentheses like (\d{4}), both groups part of a pattern and stores whatever it matched so you can reference it later by number, or by name with the (?<year>\d{4}) syntax. A non-capturing group, written (?:...), groups a portion of the pattern for applying a quantifier or alternation without saving the matched text, which keeps your group numbering clean and is slightly more efficient. Use capturing groups when you need to extract or reuse a value in a replacement; use non-capturing groups purely for structure. Both kinds can be nested, and quantifiers attach to either, so (?:ab)+ repeats the pair without capturing it. This tool lists every numbered and named capture group alongside the value it pulled from your test string, highlighting each one in the matched text, so you can confirm your groups extract exactly the data you expect before wiring them into code.

Backreferences let a replacement reuse text captured by your pattern's groups. In replacement strings you reference the first capture group as $1, the second as $2, and so on, so a pattern like (\w+)@(\w+) with the replacement $2.$1 swaps the two halves of a token around the at sign. This is how you reformat dates, reorder names, or rewrite URLs without retyping the matched content. Named groups can also be referenced depending on the syntax your target language supports. Keep in mind that inside the pattern itself a backreference like \1 instead matches the same text the group already captured, which is different from using $1 in the replacement. The find-and-replace mode in this tool previews the substituted output live as you edit the replacement string, with $1, $2, and so on resolving exactly as they would in real code. Switch to the replace tab above to test your substitution before pasting it into your project.

Regex flavors are not identical across languages. This tool runs the native JavaScript (ECMAScript) engine, the same one used in Node and browsers, but Python's re, Java's Pattern, PHP's PCRE, Go's RE2, and others differ in lookbehind support, named-group syntax, inline flag handling, and how flags are passed. A pattern that works here can need small adjustments elsewhere, especially around Unicode and backreferences. RE2-based engines like Go's even reject some constructs entirely to guarantee linear time. That is why the built-in code generator outputs your finished expression for JavaScript, TypeScript, Python, Java, PHP, Go, Ruby, C#, Rust, and Swift, translating flags into each language's idiom, such as re.IGNORECASE or Pattern.CASE_INSENSITIVE, and noting the quirks each one carries. The safest workflow is to validate against representative sample text here first, since a pattern that looks correct can still miss edge cases in another flavor. Tune your pattern above, then copy the generated code for your target language to avoid flavor surprises.