Developer Tools
Regex Tester
Regex Tester: regex Tester runs your regular expression against the sample text in the browser and highlights the matches and groups. Worked through below with real figures, and the calculator repeats the working on your own inputs — free, on-device, instant.
Enter your regular expression pattern, flags and test text and Regex Tester works out the matches and captured groups with in-browser pattern matching — fast, free and private, with no sign-up or uploads.
Runs locally in your browser and nothing you enter is uploaded. Complex patterns can backtrack heavily and slow the page, so the pattern and test text are length-capped. Matching uses the standard JavaScript regular-expression engine.
About this tool
It matters when a format, encoding or size has to be exactly right for the system reading it. The governing relationship: Regex Tester runs your regular expression against the sample text in the browser and highlights the matches and groups. See it on real numbers: Testing the pattern \b\d{4}\b against the text Call 2026 or 12345 today returns one match: 2026 . The token 12345 is not matched because it has five digits, so there is no word boundary after the fourth digit. Enable the global flag to highlight every match across longer text. The calculator above applies the same steps to whatever you enter — computed locally, nothing sent anywhere.
How to use
- Type the regular expression pattern.
- Add any flags such as g or i.
- Paste the text to test against.
- Run the tester to count the matches.
Why use the Regex Tester
Instant
Each result is computed on your device the moment you run the tool.
Private
The text and numbers you enter stay in your browser and are never uploaded.
Free
No signup and no paywall to use any tool in this set.
Works offline
Once the page has loaded it keeps working on a weak or dropped connection.
Built for developers
Plain inputs and outputs you can copy straight into code, configs, or notes.
Honest by design
Conversions and heuristics are for everyday and learning use; verify security-critical output with trusted, official tools.
Common uses
Use it when learning regex syntax
Regex Tester in data-pipeline work
Use it when writing form validation
Use it for quick pattern checks
Use it before deploying a validation rule
Use it for everyday regex reference
Technical notes
Regex Tester runs your regular expression against the sample text in the browser and highlights the matches and groups.
Results are reference values; validate against your own environment before relying on them in production.
All arithmetic runs in your browser using double-precision floating point, so extreme inputs may show tiny rounding differences.
Worked example
Testing the pattern \b\d{4}\b against the text Call 2026 or 12345 today returns one match: 2026. The token 12345 is not matched because it has five digits, so there is no word boundary after the fourth digit. Enable the global flag to highlight every match across longer text.
FAQ
Can a regex freeze the browser?
A badly written pattern can backtrack heavily, so the pattern and test text are length-capped here to reduce the risk of the page locking up.
Which flags are allowed?
The standard JavaScript flags g, i, m, s, u, y, and d are accepted, and duplicates are rejected.
What does the global flag change?
With the g flag the tester finds every match in the text; without it, only the first match is returned.
Is my pattern or text uploaded?
No. Compilation and matching happen in your browser and nothing you enter is sent away.