Fast No server upload Regex/Replace

Regex Tester Online (Test & Replace Regular Expressions)

Test regex matches, capture groups, flags, and replacements instantly in your browser.

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Processing limit: test text up to 5 MB · up to 1,000 matches · automatically stops if execution exceeds 1.5 seconds
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Common patterns

Testing Regular Expressions Without Freezing the Browser

A regular expression can be logically correct and still be unsafe on large or adversarial text. The best workflow is to start with a narrow pattern, test representative edge cases, inspect matches, and only then run replacement against larger input.

Flags change the meaning of the same pattern

Global, case-insensitive, multiline, and dot-all flags can change both the number and location of matches. Record the flag set together with the pattern when debugging or sharing a Regex result.

Replacement needs capture-group review

A pattern that matches correctly can still produce wrong output if replacement groups are referenced incorrectly. Test the replacement preview on cases with optional groups, empty matches, Unicode, and line boundaries before applying it to production text.

Watch for catastrophic backtracking

Nested ambiguous quantifiers can make some JavaScript regular expressions extremely slow. The tester enforces a time boundary, but production code should still prefer simpler patterns and bounded quantifiers where practical.

Before running Regex tests

Helpful information