# Rexact (npm · rexact-mcp)

Verify, explain and audit regexes (ReDoS, engine compatibility) offline.

- Trust score: 70/100 (medium)
- Registry status: active
- Liveness: live
- Owner verified: no
- Last scored: 2026-10-17

## Components

- npm · `rexact-mcp`: 70/100 (this document), [markdown](https://verifymcp.io/servers/nicolasmar91-rexact-mcp/rexact-mcp.md), [page](https://verifymcp.io/servers/nicolasmar91-rexact-mcp/rexact-mcp)

## Channel facts

- Registry: `npm`
- Package: `rexact-mcp`
- Version: `0.1.2`
- Transport: `stdio`

## Trust breakdown

How this component scores in each security and reliability category. Every signal is checked automatically from public evidence about the published package, including repeated runs of it in an isolated sandbox, and we only credit what we can confirm. Scores are 0–100 per category. Scoring method: https://verifymcp.io/docs/scoring (what has changed: https://verifymcp.io/docs/scoring/changelog)

Scored 2026-10-17.

- **Supply Chain Security**: 98/100
  - No malware found by supply-chain analysis.
  - No known CVEs affecting this package version or its production dependencies.
  - No install/post-install scripts declared.
  - 31 of 103 dependencies flagged as unhealthy.
- **Provenance & Transparency**: 45/100
  - Source repository is publicly reachable at the declared URL.
  - Provenance check failed: no build-provenance attestation is published.
  - Clear OSI-approved license (MIT).
  - Actively maintained (last published 1 days ago).
  - Disclosure check failed: no security disclosure policy was found in the source repository.
- **Schema Quality & AI Usability**: 74/100
  - AI-judged instruction clarity (good).
  - Tool/resource definitions use about 502 tokens (~100/item across 5 items; 5 tools + 0 resources), lean.
  - Usage-examples check failed: none of the tools include examples.
- **Stability & Change Management**: 0/100
  - Stability not yet verified: not enough scan history yet (needs a 30-day window).
- **Tool Coverage**: 100/100
  - 100% of tools have a non-trivial description (not blank, and not just the tool's name).
  - 100% of tool parameters carry a description.
  - Structured output schemas are declared (100% of tools); any adoption earns full credit.
- **Tool Safety**: 100/100
  - No prompt-injection markers were found in the server instructions, tool names or descriptions we captured.
  - We read all 5 captured tool definition(s), and no name or description among them implies an irreversible operation.
  - An AI judge read all 5 captured unit(s) of tool text and found none that tries to manipulate the model reading it.
- **Capabilities**: 100/100
  - Implements a supported MCP spec version (2025-11-25); the latest is 2026-07-28.

**Unverified: 1 category.** A category scored 0 because we could not verify it: a data source with nothing on this package, evidence we could not reach, or a check we could not run. We only credit what we can confirm.

## Install

### How do I install the Rexact MCP server?

Rexact runs locally as an npm package, launched with npx -y rexact-mcp. Ready-made configuration for Claude, Cursor, VS Code, Codex and 5 more is on this page, copied from each client's own documentation.

### Claude

```bash
claude mcp add nicolasmar91-rexact-mcp -- npx -y rexact-mcp
```

### Cursor

```json
{
  "mcpServers": {
    "nicolasmar91-rexact-mcp": {
      "command": "npx",
      "args": [
        "-y",
        "rexact-mcp"
      ]
    }
  }
}
```

### VS Code

```json
{
  "servers": {
    "nicolasmar91-rexact-mcp": {
      "command": "npx",
      "args": [
        "-y",
        "rexact-mcp"
      ]
    }
  }
}
```

### Codex

```bash
codex mcp add nicolasmar91-rexact-mcp -- npx -y rexact-mcp
```

### opencode

```json
{
  "$schema": "https://opencode.ai/config.json",
  "mcp": {
    "nicolasmar91-rexact-mcp": {
      "type": "local",
      "command": [
        "npx",
        "-y",
        "rexact-mcp"
      ],
      "enabled": true
    }
  }
}
```

### OpenClaw

```bash
openclaw mcp add nicolasmar91-rexact-mcp --command npx --arg -y --arg rexact-mcp
```

### Hermes

```yaml
mcp_servers:
  nicolasmar91-rexact-mcp:
    command: "npx"
    args: ["-y", "rexact-mcp"]
```

### Netclaw

```json
{
  "McpServers": {
    "nicolasmar91-rexact-mcp": {
      "Transport": "stdio",
      "Command": "npx",
      "Arguments": [
        "-y",
        "rexact-mcp"
      ]
    }
  }
}
```

### Vellum

```bash
assistant mcp add nicolasmar91-rexact-mcp -t stdio -c npx -a -y rexact-mcp
```

### Other

```json
{
  "mcpServers": {
    "nicolasmar91-rexact-mcp": {
      "command": "npx",
      "args": [
        "-y",
        "rexact-mcp"
      ]
    }
  }
}
```

## Changelog

Every change recorded for this component, newest first. Days that predate change tracking, or that we cannot explain, say so: "we were watching and nothing happened" and "we were not watching" are different claims.

### 2026-10-16 (score 70, +15)

- [security improvement] Malware scan: unverified → pass

### 2026-10-15 (score 55)

First indexed and scored.

## MCP tools (5)

### `verify_regex` (~90 tokens)

Verify a regex against test cases

Runs a regular expression against a set of test cases in a sandboxed worker (never on the calling process) and returns a pass/fail report. Offline, no API key required.

Input parameters:

- `cases` (array, required): Test cases to run the pattern against.
- `flags` (string): JavaScript regex flags, e.g. "gi".
- `pattern` (string, required): The regex source, without delimiters.

Output parameters:

- `durationMs` (number)
- `passed` (number)
- `results` (array)
- `total` (number)

### `check_redos` (~70 tokens)

Check a regex for catastrophic backtracking (ReDoS)

Statically and dynamically analyzes a pattern for ReDoS risk — never executes it against user-supplied input. Offline, no API key required.

Input parameters:

- `flags` (string): JavaScript regex flags, e.g. "gi".
- `pattern` (string, required): The regex source, without delimiters.

Output parameters:

- `attackString` (string)
- `complexity` (string)
- `hotspot` (array)
- `maxSteps` (number)
- `status` (string)

### `explain_regex` (~86 tokens)

Explain a regex token by token

Breaks a pattern down into an ordered list of tokens (anchors, groups, quantifiers...) with a plain-language explanation for each. Offline, no API key required.

Input parameters:

- `flags` (string): JavaScript regex flags, e.g. "gi".
- `locale` (string): Language for the plain-text explanation.
- `pattern` (string, required): The regex source, without delimiters.

Output parameters:

- `tokens` (array)

### `check_compat` (~141 tokens)

Check cross-engine regex compatibility

Reports syntax and semantic differences when a pattern is ported from its source flavor to one or more target engines (PCRE, RE2, Python, Java, .NET, POSIX grep). Offline, no API key required.

Input parameters:

- `flags` (string): JavaScript regex flags, e.g. "gi".
- `pattern` (string, required): The regex source, without delimiters.
- `sourceFlavor` (string): The flavor the pattern was authored for. Not in SPEC-TECHNIQUE.md's input list for this tool — added because checkCompat() requires it; defaults to "js".
- `targets` (array, required): Engines to check compatibility against.

Output parameters:

- `issues` (array)

### `generate_regex` (~115 tokens)

Generate a regex from a natural-language description

Asks an LLM to draft a pattern from a description and test cases, then deterministically verifies it (compile, corpus, ReDoS) before returning it — never returns unverified model output. Requires ANTHROPIC_API_KEY in the MCP server's own environment (your own key, your own cost).

Input parameters:

- `cases` (array, required): Test cases the generated pattern must satisfy.
- `description` (string, required): Natural-language description of the pattern to generate.
- `flavor` (string, required): Target regex flavor for the generated pattern.

Output parameters:

- `attempts` (number)
- `explanation` (array)
- `flags` (string)
- `flavor` (string)
- `pattern` (string)
- `redos` (object)
- `report` (object)

## Diagnostics

Captured diagnostic sections: Provenance, Dependencies. The full working is on the page: https://verifymcp.io/servers/nicolasmar91-rexact-mcp/rexact-mcp#diagnostics

## Score history

- 2026-10-17: 70
- 2026-10-16: 70
- 2026-10-15: 55

## Common questions

### What is the Rexact MCP server?

Rexact is an MCP server listed in the public MCP registry as io.github.Nicolasmar91/rexact-mcp. Verify, explain and audit regexes (ReDoS, engine compatibility) offline. This page covers its npm package (rexact-mcp).

### Is the Rexact MCP server safe to use?

Rexact scores 70 out of 100 on VerifyMCP. We found no known CVEs affecting it as of 17 October 2026. It declares no install or post-install scripts. That is a record of what we were able to check automatically, not an endorsement. The category breakdown on this page shows every signal behind the number, including the ones we could not confirm.

### What tools does the Rexact MCP server expose?

Rexact exposes 5 tools: verify_regex, check_redos, explain_regex, check_compat, generate_regex. Their descriptions and schemas cost roughly 502 tokens of context every time the server is loaded.

### Is the Rexact MCP server still maintained?

Rexact is still listed as active in the MCP registry. We last reached this channel on 17 October 2026. Those dates come from our own scans of the registry and the channel itself, not from anything the publisher announced.

### What licence is the Rexact MCP server under?

Rexact declares the MIT licence, which is OSI-approved. That covers the source only, and says nothing about the cost of any service it calls.

## Links

- npm package: https://www.npmjs.com/package/rexact-mcp
- Socket report: https://socket.dev/npm/package/rexact-mcp
- Repository: https://github.com/Nicolasmar91/rexact-mcp
- Website: https://rexact.dev/en/mcp
- Changelog RSS feed: https://verifymcp.io/servers/nicolasmar91-rexact-mcp/rexact-mcp.xml
- Changelog JSON feed: https://verifymcp.io/servers/nicolasmar91-rexact-mcp/rexact-mcp.json
- HTML version of this page: https://verifymcp.io/servers/nicolasmar91-rexact-mcp/rexact-mcp