RegexForge

An MCP (Model Context Protocol) server that synthesizes production-grade regexes from labeled examples. Zero LLM at serve time — pure symbolic synthesis over a template bank with a character-class inference fallback. Every response includes a proof matrix and a backtracking-risk audit.

• MCP transport: HTTP (streamable) · Protocol version: 2024-11-05
• MCP endpoint: https://regexforge.jason-12c.workers.dev/mcp
• MCP manifest: https://regexforge.jason-12c.workers.dev/.well-known/mcp.json

---

The MCP Tool

regexforge exposes a single MCP tool. An AI client (Claude Desktop, Cline, Continue, Cursor, or any MCP-aware agent) calls it the same way it would call any other MCP tool — tools/call over JSON-RPC 2.0.

regexforge_synth

Synthesize a battle-tested regex from labeled examples.

Input schema (what the model supplies):

{
  "type": "object",
  "required": ["examples"],
  "properties": {
    "description": {
      "type": "string",
      "description": "Optional natural-language description of the target pattern. Used only for tie-breaking when multiple templates fit."
    },
    "examples": {
      "type": "array",
      "minItems": 2,
      "maxItems": 100,
      "items": {
        "type": "object",
        "required": ["text", "match"],
        "properties": {
          "text":  { "type": "string", "maxLength": 2048 },
          "match": { "type": "boolean", "description": "true if the regex should match this string; false if it should NOT match." }
        }
      }
    }
  }
}

Output schema:

{
  "regex": "string",
  "flags": "string",
  "source": "template | char_class",
  "template_name": "string (if source=template)",
  "test_matrix": [
    { "text": "string", "expected": "boolean", "actual": "boolean", "pass": "boolean" }
  ],
  "all_pass": "boolean",
  "backtrack_risk": "none | low | high",
  "backtrack_reasons": [ "string" ],
  "candidates_considered": "integer",
  "candidates_passing": "integer",
  "notes": [ "string" ]
}

Errors return JSON-RPC error objects with structured remediation:

• not_expressible (HTTP 422) — examples imply a non-regular language (balanced-parens, counting, etc.).
• no_credits (HTTP 402) — wallet empty, purchase via /v1/credits.
• missing_input (HTTP 400) — fix field tells you exactly what's missing.

---

Connecting from MCP Clients

Claude Desktop

Add to ~/Library/Application Support/Claude/claude_desktop_config.json (macOS) or %APPDATA%\Claude\claude_desktop_config.json (Windows):

{
  "mcpServers": {
    "regexforge": {
      "transport": {
        "type": "http",
        "url": "https://regexforge.jason-12c.workers.dev/mcp"
      },
      "headers": {
        "Authorization": "Bearer YOUR_API_KEY_HERE"
      }
    }
  }
}

Get an API key with: curl -X POST https://regexforge.jason-12c.workers.dev/v1/keys (free, 50 credits on signup).

Python (official mcp SDK)

from mcp import ClientSession
from mcp.client.streamable_http import streamablehttp_client

async def main():
    url = "https://regexforge.jason-12c.workers.dev/mcp"
    headers = {"Authorization": "Bearer YOUR_API_KEY"}
    async with streamablehttp_client(url, headers=headers) as (read, write, _):
        async with ClientSession(read, write) as session:
            await session.initialize()
            tools = await session.list_tools()
            result = await session.call_tool(
                "regexforge_synth",
                arguments={
                    "description": "ISO 8601 date like 2024-12-30",
                    "examples": [
                        {"text": "2024-12-30", "match": True},
                        {"text": "2023-01-01", "match": True},
                        {"text": "12/30/2024", "match": False},
                        {"text": "abc",        "match": False},
                    ],
                },
            )
            print(result.content[0].text)

TypeScript (official @modelcontextprotocol/sdk)

import { Client } from "@modelcontextprotocol/sdk/client/index.js";
import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js";

const transport = new StreamableHTTPClientTransport(
  new URL("https://regexforge.jason-12c.workers.dev/mcp"),
  { requestInit: { headers: { Authorization: "Bearer YOUR_API_KEY" } } }
);
const client = new Client({ name: "demo", version: "1.0.0" }, { capabilities: {} });
await client.connect(transport);

const res = await client.callTool({
  name: "regexforge_synth",
  arguments: {
    description: "ethereum wallet address",
    examples: [
      { text: "0x8ABCE477e22B76121f04c6c6a69eE2e6a12De53e", match: true },
      { text: "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913", match: true },
      { text: "0x123", match: false },
      { text: "xyz",   match: false },
    ],
  },
});
console.log(res.content[0].text);

Raw JSON-RPC over HTTP

If you'd rather speak the protocol directly:

# 1. initialize
curl -X POST https://regexforge.jason-12c.workers.dev/mcp \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_KEY" \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize"}'

# 2. list tools
curl -X POST https://regexforge.jason-12c.workers.dev/mcp \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_KEY" \
  -d '{"jsonrpc":"2.0","id":2,"method":"tools/list"}'

# 3. call the tool
curl -X POST https://regexforge.jason-12c.workers.dev/mcp \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_KEY" \
  -d '{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"regexforge_synth","arguments":{"examples":[{"text":"2024-12-30","match":true},{"text":"abc","match":false}]}}}'

---

What happens under the hood

1. Template bank match — tests ~65 pre-compiled battle-tested regex templates (email, UUID v4, ISO date, semver, ETH address, US phone, SHA-256, base64, US zip, MAC address, etc.) against every example. Any template that classifies all examples correctly is a candidate.
2. Tie-break — if multiple templates pass, picks the one whose keywords best match the caller's description, breaking further ties by pattern length.
3. Character-class inference fallback — if no template fits, extracts the longest common prefix and suffix from the positive examples, infers the middle as a union of character classes [a-z0-9-]{n,m} with length bounds, then verifies the synthesized pattern rejects every negative example.
4. Return with proof — the response includes the full test_matrix so the caller (model) can verify every example classifies correctly before using the regex in code.
5. Backtracking audit — a static pass over the returned regex flags nested quantifiers, backreferences, and lookaround that might cause catastrophic backtracking.

All deterministic. No LLM call at serve time. Typical latency <100 ms.

---

Example agent workflow

An AI agent writing code that needs to parse user-supplied strings calls regexforge_synth instead of generating the regex itself (which weaker models get wrong constantly):

Model thinks: "I need to parse this SKU-1234-AB pattern. Let me not hallucinate a regex."

Calls: regexforge_synth({ description: "SKU like SKU-1234-AB", examples: [<3 positives, 5 negatives>] })

Gets back: { regex: "^SKU-[-0-9A-Z]{7}$", all_pass: true, backtrack_risk: "none", source: "char_class" }

Pastes "^SKU-[-0-9A-Z]{7}$" into its code with confidence that every known example classifies correctly.

This is a single tool call instead of: (a) LLM writes a regex, (b) LLM writes test cases, (c) LLM simulates regex execution, (d) LLM second-guesses and rewrites, … which burns 10x the tokens and still gets it wrong.

---

Auth & pricing

• Programmatic key issuance: POST /v1/keys → { key, credits: 50 }. No email. No captcha. Agents mint their own.
• Per-call cost: $0.002. Packs: starter ($5 / 2,500), scale ($50 / 30k), bulk ($500 / 350k).
• Payment (agent-autonomous): POST /v1/credits { "pack": "starter" } returns a real Stripe Checkout URL + x402 USDC-on-Base headers. Complete payment, then POST /v1/credits/verify { "session_id": "cs_..." } credits the key.
• Every error response includes a structured fix field telling the agent exactly what to change.

---

Other discovery surfaces (agent-only, machine-readable)

Endpoint	Format
GET /.well-known/ai-plugin.json	OpenAI plugin manifest
GET /.well-known/mcp.json	MCP server manifest (tools + transport)
GET /llms.txt	llms.txt standard
GET /openapi.json	OpenAPI 3.1
GET /v1/pricing	Machine-readable pricing
GET /v1/errors	Full error code catalog
GET /	Root index of all of the above

---

Implementation

• Transport: HTTP streamable (MCP spec 2024-11-05), JSON-RPC 2.0
• Deployment: Cloudflare Workers (cold start <10 ms)
• Built on: @walko/agent-microsaas — a skeleton that handles the MCP transport, discovery manifests, bearer-key auth, and credit ledger. regexforge itself is ~300 LOC of pure synthesis logic.

License

Apache-2.0.