@mukundakatta/regex-test-mcp

MCP server for trustworthy regex testing. Three tools: test_regex, find_all, replace. All use the actual JS regex engine, so you get exact semantics for lookahead, anchors, named groups, and Unicode escapes.

Why this exists

LLMs reliably hallucinate regex behavior. Common ways they're wrong:

• Confidently stating a pattern matches when it doesn't (or vice versa)
• Wrong group indices when there are nested capture groups
• Forgetting that . excludes newlines without the s flag
• Hand-rolling replace results that miss edge cases ($& semantics, named-group syntax)

Routing the actual test through this MCP server avoids all of those.

Tools

| Tool | Returns | |---|---| | test_regex(pattern, text, flags?) | {matched, match, start, end, groups, named_groups} for the first match | | find_all(pattern, text, flags?) | All non-overlapping matches (always behaves as g); zero-width safe | | replace(pattern, text, replacement, flags?) | New string + replaced boolean; supports $1, $<name>, $& |

Install — Claude Desktop

{
  "mcpServers": {
    "regex-test": { "command": "npx", "args": ["-y", "@mukundakatta/regex-test-mcp"] }
  }
}

Same shape for Cursor / Cline / Windsurf / Zed.

License

MIT OR Apache-2.0.