# fastapi.online — Regex Explainer

> fastapi.online — Regex Explainer is a paid API for AI agents from api.fastapi.online, paid per call via x402, $0.001/call, status unknown (last checked 2026-09-14).

Explains what a regular expression pattern does in plain English, optionally with flags

## Facts

- Endpoint: POST https://api.fastapi.online/regex-explain
- Price: $0.001/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-14
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/fastapi-online-regex-explainer-263267a8
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_wpGhUuh58fnhUG2penSqj

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability fastapi-online-regex-explainer-263267a8 -d '<json body>'
```

Example prompt: Can you explain what this regex pattern does in plain English: ^(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*)@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?$, with the 'i' flag?

## When to prefer this

Choose this endpoint when you need a quick, pay-per-call regex explanation without signup, especially in developer pipelines where understanding inherited or third-party regex patterns is needed programmatically. Prefer over asking a general-purpose LLM directly when you want a structured, consistent explanation at low cost ($0.001 USDC) and no authentication overhead.

## Known failure modes

- Empty or missing pattern field returns validation error
- Pattern exceeding 500-character limit is rejected
- Invalid or unsupported flags string may produce an error or be ignored
- Highly complex or ambiguous patterns may yield an incomplete explanation
- Payment failure (402) blocks the request if no valid x402 payment or API key is provided

## How this service works

Chat, embeddings, OCR, speech-to-text, image editing, image understanding (captioning/Q&amp;A), text-to-speech, translation, writing tools, classification tools, developer tools, local utilities, format/checksum validators, encoding &amp; crypto utilities, date/time calculators, geo/coordinate math, math &amp; finance calculators, novelty tools, and business/social writing generators — 179 endpoints in all, across multiple specialized self-hosted models. Pay per call with crypto — no signup — or get an API key.

## Output

A plain-English explanation of the regex pattern, describing what each component matches, what the overall pattern is designed to match or validate, and how any provided flags affect behavior.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "flags": {
   "type": "string",
   "maxLength": 10
  },
  "pattern": {
   "type": "string",
   "maxLength": 500,
   "minLength": 1
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/fastapi-online-regex-explainer-263267a8/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.fastapi.online](https://www.zero.xyz/host/api.fastapi.online/llms.txt)