Which regex flavor does it use?

It uses JavaScript (ECMAScript) regular expressions — the same engine that runs in every browser and in Node.js. Syntax is almost identical to PCRE, with a few differences in lookbehind, named groups, and Unicode property escapes.

What do the flags mean?

g finds all matches instead of just the first. i makes matching case-insensitive. m makes ^ and $ match the start and end of each line. s makes . match newline characters too. u enables full Unicode mode. y makes the regex match only at the exact lastIndex position.

Why does my regex find no matches?

Common causes: missing the g flag (only first match shown), escaping issues (\d vs d), or the pattern not matching the test text. Try toggling flags or simplifying the pattern to isolate the problem.

Is my data sent to any server?

No. The regex engine runs entirely in your browser. Your pattern and test text never leave your device — nothing is logged, stored, or transmitted.

What is catastrophic backtracking?

A performance failure where certain regex patterns (usually nested quantifiers like (a+)+) explode exponentially on non-matching input. A short string can freeze the browser for minutes. Fix it by making quantifiers atomic or rewriting the pattern to avoid nested repetition.

Are regexes the right tool for parsing HTML?

Almost never. HTML is not a regular language — nested tags, attributes and comments cannot be handled correctly by a regex. Use a proper HTML parser (DOMParser in the browser, or a library like Cheerio, BeautifulSoup, or jsdom) for reliable extraction.

Regex Tester

Test JavaScript regular expressions with live highlighting. Matches appear as you type. Toggle all six flags — g, i, m, s, u, y — independently. See index, full match and capture groups for every match. Clear error messages for malformed patterns.

How to use

Enter your regex

Type the pattern between the slashes or in the pattern field.

Toggle flags

Enable g, i, m, s, u or y as needed.

Paste your test text

Drop in the text you want to match against.

Review matches

See all matches highlighted, with index and capture groups listed below.

Regular expression

//g

2 matches

Test string

Highlighted matches

Contact us at hello@example.com or support@mycompany.org.
Invalid: not-an-email

Match details

#1Index: 14

Full match:hello@example.com

#2Index: 35

Full match:support@mycompany.org

Features

Live highlighting

Matches are highlighted in the test text as you type.

Match details

See the index, full match and capture groups for every match.

Six flags

g, i, m, s, u and y — toggle them independently.

Invalid pattern feedback

Clear error messages if the regex is malformed.

Common use cases

Debug a regex before adding it to code
Test a pattern against sample data
Check capture groups and match indices
Learn regex syntax with live feedback

Frequently Asked Questions

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