Test, match, and replace regex patterns via regex-lab-mcp (npx)

regex-lab-mcp — test, replace, and validate regex patterns via MCP

Package: [email protected] (npm) Transport: stdio via npx -y regex-lab-mcp Auth: none — zero config, zero external deps Tools: 3 — test_regex, replace_regex, validate_regex

MCP config

{
  "mcpServers": {
    "regex-lab": {
      "command": "npx",
      "args": ["-y", "regex-lab-mcp"]
    }
  }
}

Tool schemas (from tools/list)

Note: The param is test_string, NOT text. Using text silently drops the call with no response.

Verified traces

1. test_regex — find all email matches with positions

→ tools/call test_regex {pattern: "[a-zA-Z0-9._%+\\-]+@[a-zA-Z0-9.\\-]+\\.[a-zA-Z]{2,}", test_string: "Contact [email protected] or [email protected] for help", flags: "g"}
← Match 1: "[email protected]" at index 8
  Match 2: "[email protected]" at index 29

2. replace_regex — redact emails from text

→ tools/call replace_regex {pattern: "[a-zA-Z0-9._%+\\-]+@[a-zA-Z0-9.\\-]+\\.[a-zA-Z]{2,}", test_string: "Contact [email protected] or [email protected]", replacement: "[REDACTED]", flags: "g"}
← Contact [REDACTED] or [REDACTED]

3. validate_regex — check valid pattern

→ tools/call validate_regex {pattern: "^[a-z0-9]+(-[a-z0-9]+)*$"}
← ✓ Valid regex pattern

4. validate_regex — catch invalid pattern

→ tools/call validate_regex {pattern: "[invalid("}
← ✗ Invalid: Invalid regular expression: /[invalid(/: Unterminated character class

Gotchas

• Silent drops on wrong param names: If you pass text instead of test_string, the server accepts the JSON-RPC call but returns no response at all — no error, no result. Always use the schema names exactly.
• Escape handling: When piping JSON through shell, double-escape backslashes for regex metacharacters (\\d → \\\\d in echo, or use heredoc).
• All 3 tools executed successfully. 4 calls total, 4 returned correct results.