# POST /regex/explain: $0.001 per call

Validate and explain a regular expression (JavaScript flavour): is it valid, what each part means in plain English, whether it risks catastrophic backtracking (ReDoS), and what it matches in up to 20 test strings (with groups). POST {pattern, flags, tests}.

- **Price:** $0.001 in USDC, the same on Base, Solana, Polygon, Arbitrum. Failed calls are never charged.
- **Free trial:** yes, 20 free calls a day from Claude, Cursor or any MCP client ([set-up](/mcp/setup)).
- **Tier:** live
- **Answers cached for:** 1 day(s)
- **Data sources:** Local analysis (no upstream calls)
- **Lane:** AI coding agents ([OpenAPI](/openapi/coding.json))
- **Live health:** [status page](/status)

## Free sample

See an answer for the demo input first, free (no payment, 10 a minute): [https://aayatai.com/sample/regex-explain](/sample/regex-explain). It is a stored real answer when we have one, otherwise an example marked `"kind": "illustrative"`.

```bash
curl "https://aayatai.com/sample/regex-explain"
```

## 1. See the price (free)

Call it without paying: you get `402 Payment Required` and a `PAYMENT-REQUIRED` header with the exact price and where to pay.

```bash
curl -i -X POST https://aayatai.com/regex/explain -H "content-type: application/json" -d '{"pattern":"^(\\d{3})-(\\d{4})$","flags":"","tests":["555-1234","5551234"]}'
```

## 2. Pay and call (TypeScript)

```bash
npm install @x402/fetch @x402/evm viem
```

```ts
import { wrapFetchWithPaymentFromConfig } from "@x402/fetch";
import { ExactEvmScheme } from "@x402/evm";
import { privateKeyToAccount } from "viem/accounts";

// A wallet used only by your agent, holding a little USDC on Base.
const account = privateKeyToAccount(process.env.WALLET_PRIVATE_KEY as `0x${string}`);
const pay = wrapFetchWithPaymentFromConfig(fetch, {
  schemes: [{ network: "eip155:8453", client: new ExactEvmScheme(account) }],
});

const res = await pay("https://aayatai.com/regex/explain", {
  method: "POST",
  headers: { "content-type": "application/json" },
  body: JSON.stringify({"pattern":"^(\\d{3})-(\\d{4})$","flags":"","tests":["555-1234","5551234"]}),
});
console.log(await res.json());
```

## 3. Or as an MCP tool

```ts
// MCP server: https://aayatai.com/mcp (Streamable HTTP). With the x402 MCP client (see /start):
const result = await client.callTool("regex-explain", {"pattern":"^(\\d{3})-(\\d{4})$","flags":"","tests":["555-1234","5551234"]});
```

## Inputs

- `pattern` **(required)** (string): The regex source, without slashes, e.g. ^(\d{3})-(\d{4})$
- `flags` (string): JavaScript flags: g i m s u y d v.
- `tests` (array): Strings to try it on.

Bad inputs are rejected with HTTP 400 before any payment is asked for.

## Example answer

```json
{
  "valid": true,
  "error": null,
  "pattern": "^(\\d{3})-(\\d{4})$",
  "flags": "",
  "parts": [
    {
      "token": "^",
      "meaning": "start of the text (or line with the m flag)"
    },
    {
      "token": "(",
      "meaning": "start of capturing group"
    },
    {
      "token": "\\d",
      "meaning": "a digit (0-9)"
    },
    {
      "token": "{3}",
      "meaning": "repeat the previous item exactly 3 times"
    }
  ],
  "groups": 2,
  "risk": null,
  "tests": [
    {
      "input": "555-1234",
      "matched": true,
      "matches": [
        {
          "match": "555-1234",
          "index": 0,
          "groups": [
            "555",
            "1234"
          ],
          "named": null
        }
      ]
    },
    {
      "input": "5551234",
      "matched": false,
      "matches": []
    }
  ]
}
```

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- [POST /docs/answer](/services/docs-answer) ($0.02): Ask a coding question about any npm, PyPI, crates or Go library and get an answer written only from its current docs (project llms.txt, docs pages or latest REA
- [GET /openapi](/services/openapi) ($0.005): Understand any public API fast: give its OpenAPI/Swagger spec URL, its docs or base URL, or a name from the APIs.guru directory (?api=stripe.com); we find the s
- [GET /library/research](/services/library-research) ($0.10): Premium research report on a library or API for coding agents: reads its current docs for your goal, checks version, safety and repository health, gathers what 
- [GET /package/check](/services/package-check) ($0.005): Should a coding agent install this package?
- [GET /dependency/verdict](/services/dependency-verdict) ($0.03): "Should I use this dependency?" in one call for npm, PyPI, crates or Go: full package safety check (vulnerabilities, malware, typosquats, deprecation, licence, 
- [GET /dependency/report](/services/dependency-report) ($0.08): Premium "should I use this dependency?" report: package safety (vulnerabilities, malware, typosquats, deprecation), GitHub repository health, licence compatibil

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