Which regex flavour does this use?

JavaScript (ECMAScript) regular expressions, the same engine your browser uses to run JS. Syntax follows the ECMAScript 2023 specification, including lookbehinds, named groups, and Unicode property escapes (with the u flag).

Which flags are supported?

Six JavaScript flags: g (global), i (case-insensitive), m (multiline), s (dotAll), u (Unicode, needed for \p{...} property escapes and \u{...} code point escapes), and y (sticky). Toggle each flag with a button next to the pattern input.

What happens if my regex is invalid?

An "Invalid regular expression" message appears next to the pattern input. The page stays responsive and you can edit the pattern until it parses.

Is my text sent to a server?

All processing runs in your browser using the native JavaScript RegExp engine. Our servers are not involved at any point.

JavaScript Regex Tester

Test JavaScript regular expressions against your text and see every match listed as you type. The tester uses the browser's built-in RegExp engine, so behaviour matches exactly what your JavaScript code will see at runtime. Runs entirely in your browser.

Test string

0 matches

Enter a pattern and test string above

MODE

Runs entirely in your browser

How it works

The tool uses the browser's native JavaScript RegExp engine. Patterns are compiled with new RegExp(pattern, flags) and run in a Web Worker that is stopped after 2 seconds, so a runaway pattern cannot freeze the page. Invalid patterns show an error message next to the pattern and never crash the page.

Processing runs in your browser

Both the pattern and the test string are processed entirely in your browser. Our servers are not involved at any point.

Related operations

For plain substring swaps without regex, try find and replace. To validate JSON patterns you are matching, use the JSON formatter. For decoding escape sequences in HTML attribute values, see HTML entities.

Frequently asked questions

Which regex flavour does this use?: JavaScript (ECMAScript) regular expressions, the same engine your browser uses to run JS. Syntax follows the ECMAScript 2023 specification, including lookbehinds, named groups, and Unicode property escapes (with the u flag).
Which flags are supported?: Six JavaScript flags: g (global), i (case-insensitive), m (multiline), s (dotAll), u (Unicode, needed for \p{...} property escapes and \u{...} code point escapes), and y (sticky). Toggle each flag with a button next to the pattern input.
What happens if my regex is invalid?: An "Invalid regular expression" message appears next to the pattern input. The page stays responsive and you can edit the pattern until it parses.
Is my text sent to a server?: All processing runs in your browser using the native JavaScript RegExp engine. Our servers are not involved at any point.

Last reviewed September 30, 2026

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