regexguard-mcp-server

An MCP server that gives any AI agent a way to sanity-check a regex it just generated -- both what it actually matches and whether it's safe to run against untrusted input -- before shipping it. Fully local: no API key, no network call, no dependency beyond the MCP SDK and Zod.

Why

Regexes are a notoriously easy place to introduce a bug that looks fine in every example you happen to test. Two failure modes in particular are both common and easy to miss by eye:

1. It doesn't match what you think it matches. explain_regex turns the pattern into a real syntax tree and describes it in plain English, so "does this actually require at least one digit?" has a fast answer that doesn't depend on trusting your own reading of nested brackets.

2. It's a denial-of-service vector. A regex with nested quantifiers ((a+)+) or ambiguous alternation inside a repeated group ((a|a)+) can make a backtracking engine take exponential time on a crafted (or even accidental) non-matching input -- this is ReDoS, a real and repeatedly-exploited vulnerability class, and a plausible defect in any regex an agent writes without testing it against adversarial input. check_redos_risk flags the structural shape without ever executing the pattern -- it's safe to run on untrusted or deliberately malicious regex source.

Related MCP server: Regex Toolkit MCP Server

Tools

explain_regex

Parses a pattern into an AST and returns a plain-English description of what it matches.

check_redos_risk

Statically analyzes a pattern's structure for nested quantifiers and ambiguous alternation inside a repeated group -- the two classic causes of catastrophic backtracking. Returns "safe", "high", or "critical", with a specific finding for each issue found.

Both tools share one parser (src/services/parser.ts): a real recursive-descent regex parser (literals, character classes, shorthand classes, anchors, capturing/non-capturing/named groups, lookaround, alternation, quantifiers, backreferences), not a bag of string-matching heuristics against the pattern's raw source text.

This is a heuristic structural check, not a formal verifier -- check_redos_risk can tell you a pattern has the textbook exponential-blowup shape; it can't prove a pattern is fast on all inputs, and there are ReDoS patterns outside the two shapes it currently detects. Treat a "safe" result as "nothing obvious found," not a guarantee.

Install and configure

git clone https://github.com/wedo911/regexguard-mcp-server.git
cd regexguard-mcp-server
npm install
npm run build

Add it to your MCP client's config (e.g. claude_desktop_config.json, or a project's .mcp.json for Claude Code):

{
  "mcpServers": {
    "regexguard": {
      "command": "node",
      "args": ["/absolute/path/to/regexguard-mcp-server/dist/index.js"]
    }
  }
}

Run the tests

npm run build
node --test tests/parser.test.mjs tests/explain.test.mjs tests/redosCheck.test.mjs

44 tests cover the parser grammar, the explanation output, and both true positives ((a+)+, (a*)*, (a|a)+, (a|ab)+, patterns nested inside non-capturing groups) and true negatives ((cat|dog)+, a realistic username pattern, a realistic email pattern, sibling — not nested — repetitions) for the ReDoS check, so the false-positive rate on ordinary patterns is a tested property, not a hope.

Try it without a client

npx @modelcontextprotocol/inspector --cli node dist/index.js \
  --method tools/call --tool-name check_redos_risk \
  --tool-arg pattern='^(([a-zA-Z0-9])+([\.-]?([a-zA-Z0-9])+)*)$'

License

MIT — see LICENSE.