Which regex engine does this use?

It uses JavaScript RegExp in the browser. Other engines may support different flags, escapes, and replacement syntax.

Do I include the slashes?

No. Enter only the pattern body. For example, use \d+ and toggle the g flag instead of typing /\d+/g.

Are named groups supported?

Yes, when the browser supports JavaScript named capture groups such as (? ...).

Can a regex freeze the page?

Some patterns can cause catastrophic backtracking on long input. Keep test input small and use timeouts in production systems.

Online Tools

Regex Tester

Test JavaScript regular expressions with flags, matches, capture groups, named groups, highlighting, and replacement preview.

Regular expression

//g

Enter only the pattern between slashes: use \\d+ instead of /\\d+/g, then toggle the g flag.

Test text

Replacement

lt;name>" value=""/>

Preview uses JavaScript replace: $1, $2, and

lt;name> follow browser RegExp rules.

Complex regular expressions on long input can slow down or briefly freeze this tab. For production validation, limit input length and execution time.

Test JavaScript regular expressions

Use this regex tester to check JavaScript RegExp patterns against sample text, inspect matches, review capture groups, and preview replacement output.

Enter the pattern without surrounding slashes.
Toggle common JavaScript flags: g, i, m, and s.
Review match indexes, numbered capture groups, and named capture groups.
Preview JavaScript replace output with $1, $2,
amp;, and
lt;name> syntax.

Browser RegExp behavior

The tool uses the browser's JavaScript RegExp engine. PCRE, Python, Java,.NET, and database regex engines may differ in lookbehind support, flags, escaping, named groups, and replacement syntax.

p a tt er n + f l a g s + t es t t e x t \to ma t c h es

pattern is the expression body, flags control matching mode, and matches are returned by JavaScript RegExp.

Performance and safety

Avoid catastrophic backtracking

Complex regular expressions on long input can slow down or freeze a browser tab. Production systems should limit input length and execution time.

The tester is useful for debugging and learning, but it is not a full regex debugger and does not guarantee identical behavior in every runtime.

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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