A pattern that looks right in a pull request often matches more, or less, than its author intended.
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Regex Tester
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Free · API · MCPToolYour platform
Use Regex Tester three ways
The free Regex Tester on this page works in your browser. The same workflow is available via REST API for apps and via MCP for Cursor, Claude, and other agents — one API key, one plan, one quota. This tool is part of the developer-apis module under Developer Tools.
- Free in browserUse Regex Tester here — no account required.
- REST APICall from your server with
X-Api-Key. - MCP for agentsConnect Cursor or Claude to ToolYour MCP with the same key.
- 1. Get an API keySign in and open Dashboard → API Keys (works for REST and MCP).
- 2. Call or connectUse the REST API from your server, or paste the MCP URL into your agent config.
- 3. Same qualityOutput matches the free web tool across all three surfaces.
What is Regex Tester?
A pattern that looks right in a pull request often matches more, or less, than its author intended. This regex tester compiles your pattern with the flags you supply and runs it against sample text, returning each match with its character index and its capture groups so you can see exactly what the expression consumed.
The engine is JavaScript's RegExp, so lookbehind, named groups, and Unicode property escapes behave as they do in Node and the browser — but PCRE, Python, RE2, and Go syntax differences are real, and a pattern verified here is verified for JavaScript. Global scans stop after 500 matches to protect the worker, and a catastrophically backtracking pattern can still be slow on hostile input, so keep an eye on nested quantifiers. If you need a reliable starting pattern instead of debugging your own, Regex Generator returns vetted presets.
Browser use is free and needs no signup. The same job runs over REST at POST https://api.toolyour.com/api/v1/developer-apis/regex-tester and over MCP at https://api.toolyour.com/mcp — one API key, one shared monthly credit pool, 1 credit per successful call.
What are common questions about Regex Tester?
Which regex flavour does it use?
JavaScript RegExp, with the standard flags g, i, m, s, u and y. PCRE and Python syntax may behave differently.
Is there a limit on matches?
Yes. A global scan returns at most 500 matches per run so a broad pattern on a large input cannot exhaust the worker.
Does it show capture groups?
Every match includes its start index and an ordered list of capture group values, which is usually where an off-by-one grouping bug becomes obvious.
What happens if the pattern will not compile?
You get the engine's compile error — unbalanced brackets, an invalid escape, or an unsupported flag — before any matching is attempted.
What are the key features of Regex Tester?
Matches with indexes
Every hit reports where it started, which makes overlapping and greedy behaviour visible.
Capture groups listed
Group values are returned per match so extraction logic can be checked directly.
Standard flag support
Pass any combination of JavaScript flags exactly as you would in code.
How do you use Regex Tester?
Enter the pattern and flags
Compile errors surface immediately if the syntax is wrong.
Paste representative text
Include the awkward cases you expect to fail, not only the happy path.
Review matches and groups
Check both what matched and what was skipped before shipping the pattern.
Testing the cases that actually break patterns
Most regex bugs reach production because the sample text was too friendly. A few categories catch the majority of them. **Boundaries** — the very first and very last position in the string, where anchors and lookarounds behave differently than in the middle. **Empty and near-empty input** — a pattern with an optional body can match an empty string repeatedly. **Greedy spans** — a
.* between two delimiters will swallow everything to the last delimiter, not the first.
Add unusual whitespace, accented characters, and a line that should not match at all. A pattern that correctly returns zero matches on negative samples is worth more evidence than one that matches your three positive examples.From pattern to production
A tested pattern is a starting point, not a finished validator. Two follow-ups matter. Performance — nested quantifiers over alternation can backtrack exponentially, so if a pattern feels slow on a long line here, it will be a denial-of-service risk on user input. Portability — if the pattern is destined for Python, Go, or a database engine, re-test it there, because named group syntax and lookbehind support differ.
For common shapes, start from Regex Generator rather than writing from scratch, then bring the preset back here and test it against your own awkward samples.
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