Before you start
- Use a representative, non-sensitive sample and state the exact output you expect.
- Keep the source text or pattern so that an unexpected result can be reproduced.
Text tool
JavaScript Regex Tester
Test regular expressions, inspect matches and groups, and preview replacements in your browser.
Test JavaScript regex as a small behavior contract
Write examples that should match and should not match, then check full matches, capture groups, flags, and replacement output together.
- This tool uses the browser JavaScript RegExp engine with g, i, m, s, u, and y flags; PCRE, Python, Java, and server-side engines may differ.
- Test text is capped at 500 KB and display stops after 1,000 matches. Replacement preview always applies globally when a replacement is provided.
- Avoid nested ambiguous repetitions such as `(a+)+` on large near-matches. There is no execution timeout, so pathological patterns can make the browser page unresponsive.
Patterns, text, results, copying, and replacement preview remain in the browser. Use a redacted sample; logs, tokens, and customer data can still be sensitive locally.
How to Test JavaScript Regular Expressions With Edge Cases and Safer ReplacementsReview rendered or tested text in context
Check the result
- Test edge cases such as empty values, line breaks, Unicode characters, and literal punctuation.
- Open rendered Markdown or replacement output in the destination where it will actually be used.
Know the limit: A preview or a successful test does not guarantee accessibility, publishing policy, or application-specific behavior.
Tool details
JavaScript Regex Tester and Replacement Preview
Test JavaScript regular expressions, flags, matches, capture groups, and replacement output locally in your browser.
What This Tool Does
This browser-based regex tester uses the JavaScript RegExp engine, highlights matches in test text, lists match indexes, shows numbered and named capture groups, and previews replacement output.
Common Use Cases
- Testing regex patterns against logs, API responses, or text snippets
- Checking numbered and named capture groups before using a pattern in code
- Previewing JavaScript replacement strings such as $1, $&, and $<name>
How Data Is Handled
Regex testing runs locally in your browser. Patterns, test text, and replacement text are not uploaded to the backend.
FAQ
Which regex syntax does this tool use?
It uses JavaScript RegExp syntax, so behavior may differ from PCRE, Python, Java, or other regex engines.
Does it support named capture groups?
Yes. Named groups are shown when the browser supports JavaScript named capture groups.
Is my test text sent to the server?
No. Regex testing runs locally in your browser.