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Safe Regex Tester

Test regular expressions, flags, offsets and capture groups with a linear-time engine.

Patterns and test text stay on this device.

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What Safe Regex Tester does

Compile and run RE2-compatible regular expressions against local test text, then inspect bounded match output, capture groups and exact UTF-16 offsets without catastrophic backtracking.

Privacy and processing

Patterns and test text stay on this device. Entered values remain in the current page session and are not included in analytics or monitoring.

How to use Safe Regex Tester

  1. 1Enter an RE2-compatible pattern and optional flags.
  2. 2Paste test text and run the expression.
  3. 3Inspect matches and groups or download the match JSON.

Common uses

  • Debugging validation patterns
  • Checking capture groups
  • Testing patterns against multiline logs

Limitations

  • Backreferences and lookaround are unsupported to preserve predictable matching performance.
  • Offsets use JavaScript UTF-16 positions and match output is capped for responsiveness.

Frequently asked questions

Why are some JavaScript regex features unsupported?

The RE2 engine omits features such as backreferences and lookaround so matching remains predictable and resists catastrophic backtracking.

Does the tool replace text?

No. It focuses on inspectable matches and groups; use Find and Replace for local replacement workflows.

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