Regex Tester
Write or paste a regular expression and watch every match light up in your test string as you type. The part most testers leave out is here too: an explanation panel that breaks your pattern down piece by piece and tells you, in plain English, what each bracket, quantifier and escape is actually doing. You also get a match inspector with capture groups, a search-and-replace preview, ready-made patterns and a cheat sheet. It runs on JavaScript's native RegExp engine, so what you see is exactly what your code will do.
- Plain-English explanation of every part of your pattern
- Live highlighting as you type, with each match coloured
- Clickable flag toggles that say what each flag does
- Match inspector with positions, capture groups and named groups
- Search-and-replace preview with $1 substitutions
- Ready-made patterns: email, URL, IPv4, CNIC, PK mobile and more
- Built-in cheat sheet for when you forget the syntax
- Native JavaScript engine — identical to Node and the browser
How to use the Regex Tester
- 01
Write or pick a pattern
Type your regex, or start from one of the ready-made patterns.
- 02
Set your flags
Click g, i, m, s, u or y — each chip explains what it changes.
- 03
Add test text
Paste the string you want to match against and watch it highlight.
- 04
Read the breakdown
Open Explanation to see what your pattern does, token by token.
Frequently asked questions
What is a regular expression (regex)?+
A regular expression is a compact way of describing a search pattern for text. Instead of looking for one fixed string, you describe a shape — three digits, then a dash, then seven digits — and the engine finds everything that fits. Regex is used everywhere for searching, validating input and reformatting text.
How does the explanation panel work?+
It walks through your pattern from left to right and describes every construct it finds: character classes, quantifiers, anchors, groups, lookarounds, back-references and escapes. Nested groups are indented so you can see the structure, and each row shows the exact snippet it is describing. It is the fastest way to understand a regex you inherited from someone else.
What do the g, i, m, s, u and y flags mean?+
g (global) finds every match instead of stopping at the first. i ignores case. m (multiline) makes ^ and $ match at each line break rather than only at the ends of the string. s (dotAll) lets the dot match line breaks too. u enables full Unicode escapes like \p{L}. y (sticky) matches only at the current position instead of searching ahead.
Why is only one match highlighted?+
Because the g flag is off. Without it, JavaScript returns the first match and stops. Click the g chip to turn global matching on and every occurrence will be highlighted.
What is the difference between (abc) and (?:abc)?+
Both group the expression so a quantifier can apply to the whole thing. (abc) also captures — it remembers what it matched so you can reference it as $1 in a replacement or in the match inspector. (?:abc) groups without capturing, which keeps your group numbers clean and is slightly faster when you do not need the captured text.
How do I use capture groups in the replacement?+
In the Replace tab, use $1, $2 and so on for numbered capture groups, $<name> for named groups like (?<year>\d{4}), and $& for the entire match. The preview updates live so you can confirm the result before pasting it into your code.
Which regex flavour does this tester use?+
JavaScript's built-in RegExp engine, running in your own browser. The behaviour is identical to what you get in Node.js, React or any modern browser. Patterns from other flavours — PCRE, Python, Java — are usually close but not always identical, so a pattern copied from another language may need small adjustments.
Is my test data saved?+
No. Everything happens locally in your browser. Your patterns and test strings are never sent to a server, so it is safe to test against real log lines or customer data.
Need a custom tool or a mobile app? Let’s build it.
I’m a React Native & web developer in Karachi. These free tools are built with the same care I bring to client work.