JavaScript regex tester

Test JavaScript regex matches, capture groups and string replacement in a cancellable worker with a time limit.

Your input

Outputs are cleared when the input changes.

Result

Enter your data or use the example, then run the tool.

Worked example

(a)(b) with g on ab ab returns two matches, each with groups a and b; $2$1 previews ba ba.

Empty matches advance safely. Catastrophic backtracking is stopped by terminating the worker.

Method

Construct RegExp only inside a dedicated worker; exec results include UTF-16 indices and capture groups.

Advance empty matches by code point in Unicode mode. Replacement uses bounded JavaScript string-token semantics, not executable functions.

Limits

Pattern at most 10,000 UTF-8 bytes, text at most 100,000 UTF-16 units, 1,000 matches and two-second worker timeout.

JavaScript syntax only, not PCRE/Python. Unsupported flags are errors; a truncated match list never produces a claimed-complete replacement.

Sources

MDN: JavaScript RegExp semantics

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