Regex Tester

What is Regex Tester?

The Regex Tester is a free online regular expression tester and debugger that runs a JavaScript regex pattern against sample text and highlights every match live as you type. Developers, data wranglers, and anyone writing search-and-replace or validation rules use it to build and debug patterns, inspect capture groups, and see how flags change matching. Enter a pattern between the slashes, toggle the g, i, m, s, u, and y flags, paste a test string, and instantly view the match count, highlighted matches, and per-match capture group values. Because it uses the browser-native RegExp engine, results behave exactly as they will in your JavaScript code.

How to use Regex Tester

1. Type your regular expression into the Regular expression field shown between the slash delimiters.
2. Toggle the flags you need under Flags: g global, i ignore case, m multiline, s dotAll, u unicode, or y sticky; g is on by default. Active flags appear after the closing slash next to your pattern.
3. Paste or type your sample text into the Test string box, or click Clear to empty it.
4. Read the Matches count and review the Highlighted panel, where every match is highlighted inline within the test string.
5. Check the Match details list to see each match's position index, the matched value, and the value of every numbered capture group.

Examples

pattern \d+ (digit class, flag g) on text a1b22c333

Matches: 3 (highlights 1, 22, 333)

pattern Hello (flags i and g) on text hello HELLO Hello

Matches: 3 (highlights hello, HELLO, Hello)

Frequently asked questions

Which regex flags are supported?

All six JavaScript flags: g (global), i (ignore case), m (multiline), s (dotAll), u (unicode), and y (sticky). Click any flag toggle to switch it on or off; the active flags appear after the closing slash next to your pattern.

What regex syntax does it use?

It uses the browser-native JavaScript RegExp engine, so your pattern behaves exactly as it would in JavaScript or TypeScript code, including capture groups, character classes, lookarounds, and Unicode escapes when the u flag is on.

Why does it say my expression is invalid?

If the pattern cannot be compiled (for example an unbalanced bracket or an unknown escape), the tool shows the exact error message returned by the RegExp constructor instead of any matches. Fix the pattern and the matches update instantly.

Does it show capture groups and match positions?

Yes. Each match is listed in Match details with its starting index in the text, the matched value, and every numbered capture group; empty groups show (empty) and groups that did not participate show a dash.

Is my pattern or test text uploaded anywhere?

No. Everything runs 100% client-side in your browser using the native RegExp engine. Your pattern and test string are never sent to a server, so you can safely test against sensitive or private data.