Regex Tester

Flags that change the result

g finds every match instead of the first, and i ignores case. The two that are most often confused: m makes ^ and $ match at every line break rather than only at the start and end of the text, while s lets . match line breaks too. u turns on full Unicode handling, needed for emoji and \p{L}-style letter classes.

Patterns that freeze

Nested quantifiers such as (a+)+$ or (\w+\s?)*$ can try an exponential number of ways to match before failing, so a single line of input freezes a server or a browser tab. This tester stops a pattern after 750 milliseconds. If yours is stopped, remove the nesting or make the inner part unable to match the same text in more than one way.

JavaScript and other regex flavours

Patterns move between languages mostly intact, with differences at the edges. JavaScript names groups with (?<name>…), Python uses (?P<name>…), and JavaScript has no possessive quantifiers or atomic groups, which PCRE, Java, and .NET support. A pattern that works here may need small changes before it runs elsewhere.

How to use it

1. Enter a pattern, flags, and test text.
2. Run the expression.
3. Review match positions and capture groups.

Privacy & limitations

Test text never leaves your browser. JavaScript regex syntax and Unicode behavior can differ from other languages.