Regex Tester

Quick answer: Regex Tester is a free online tool that lets you test JavaScript regular expressions against your text with match counts, positions, and capture groups. Invalid patterns show clear errors. It works instantly with no signup or download — everything runs privately in your browser.

Test JavaScript regular expressions against your text with match counts, positions, and capture groups. Invalid patterns show clear errors. Free, no signup, runs locally.

What is the Regex Tester?

A regex tester lets you experiment with regular expressions — compact patterns that match text — against real input, showing every match, its position, and captured groups. This regex tester online evaluates patterns with the JavaScript RegExp engine, the same engine that runs in browsers and Node.js, so a pattern that works here works in your code.

Type your pattern in the second box and your test string in the first. Flags like g (find all matches), i (case-insensitive), and m (multiline) go in the flags field. The results table lists each match with its index, so you can verify boundary cases — does \b\d{4,}\b catch the 5-digit order number but skip the year inside the date? You will see instantly.

Invalid patterns do not fail silently: you get the engine’s exact error message, which usually points at an unclosed group or a stray quantifier. Everything evaluates locally in your browser — paste log samples and production data freely with no signup and nothing uploaded.

When a pattern misbehaves, bisect it. Test half the pattern, confirm which half matches, then grow from the working side — the match table makes each step visible. Most regex bugs come from three places: an unescaped special character, a greedy quantifier eating too much, or missing anchors letting partial matches through. Check those first and you will solve the majority of issues in minutes.

How to use the Regex Tester

  1. Paste the text you want to search into the first box.
  2. Type your regular expression into the pattern box (without the surrounding / slashes).
  3. Add flags if needed: g for all matches, i to ignore case, m for multiline, s for dot-all.
  4. Click Test Regex to see the match count and a table of every match with its position.
  5. Refine the pattern and re-run until it matches exactly what you intend — then copy it into your code.

Regex Tester use cases

Validating input formats

Email checks, phone numbers, postal codes, and password rules are classic regex jobs. Test the pattern against valid and invalid samples here before shipping it in a form validator.

Extracting data from logs

Pull order IDs, timestamps, or error codes out of log lines with capture groups. The match table shows each hit and its index, making it easy to confirm the pattern against messy real data.

Building search-and-replace rules

Before running a regex replace across a codebase, verify the match set here — a greedy .* can swallow far more than intended, and the table makes over-matching obvious.

Learning regular expressions

Regex is best learned by doing. Tweak one token at a time — \d vs \w, * vs +, greedy vs lazy — and watch the match list change to build real intuition.

Tips

  • Start with the g flag so you see all matches, not just the first one.
  • Anchor with ^ and $ when the whole string must match, not just part of it.
  • Prefer lazy quantifiers (*? and +?) when a greedy match swallows too much text.
  • Test against inputs that should NOT match — a pattern is only correct if it also rejects correctly.

Frequently asked questions

Which regex flavor does this tester use?
JavaScript (ECMAScript) RegExp — the same engine in browsers and Node.js. Most syntax transfers to Python, Java, or PCRE, but advanced features like lookbehind and named groups vary, so verify in your target language for exotic patterns. Since the same engine powers form validation in browsers, patterns you perfect here drop straight into frontend code unchanged.
What do the g, i, m, s flags mean?
g finds all matches instead of stopping at the first; i makes matching case-insensitive; m makes ^ and $ match at line boundaries; s lets . match newlines too. Combine them freely, e.g. gi. The flags field accepts any combination — start with g while developing so you can see every match your pattern produces.
Why does my pattern show an error?
The engine reports the exact problem — usually an unclosed parenthesis or bracket, a quantifier with nothing to repeat (+ at the start), or an invalid escape. Fix the reported issue and test again. Pasting the engine's message into a search usually surfaces the exact syntax rule you tripped over, with examples.
How do I match the whole string, not just part of it?
Anchor the pattern: ^ at the start and $ at the end, e.g. ^\d{5}$ matches exactly five digits and nothing else. Without anchors, the engine reports a match if the pattern appears anywhere. Anchors are the single most important concept for validation patterns — without them, almost any input can partially match.
What is the difference between greedy and lazy matching?
Greedy quantifiers (*, +) grab as much as possible; lazy ones (*?, +?) grab as little as possible. For on '', greedy matches the whole string while lazy matches '' then ''. When in doubt, try the lazy version first for extraction tasks — it usually matches the intuitive interpretation of the pattern.
Is my test data uploaded?
No. The pattern compiles and runs entirely in your browser's JavaScript engine. Paste production logs freely — nothing leaves your device and there is no signup. The RegExp engine is built into your browser, so evaluation is instant and completely offline-capable after load.
Why do I only see one match?
Without the g flag, the engine stops at the first match. Add g to the flags field to list every match in the text. This trips up nearly everyone at first — the g flag is what turns a tester from single-match into find-all mode.