If you've ever built a form backend or an automation workflow,
you know the pain: you need to validate an email, a phone number,
an IBAN, a credit card, and a URL — and you end up calling five
different APIs to do it.
I built MultiValidator to fix that. One API call. Up to 50 fields.
13 format types. Here's how it works and how I built it in a weekend.
What it does
Send a batch of fields, get back validation results for all of them:
import requests
payload = {
"fields": [
{"type": "email", "value": "[email protected]", "field_name": "email"},
{"type": "phone", "value": "+447911123456", "field_name": "mobile"},
{"type": "iban", "value": "GB29NWBK60161331926819", "field_name": "bank"},
{"type": "credit_card", "value": "4111111111111111", "field_name": "card"},
{"type": "url", "value": "https://example.com", "field_name": "website"}
]
}
response = requests.post(
"https://multivalidator-batch-format-validation.p.rapidapi.com/validate/batch",
json=payload,
headers={"X-RapidAPI-Key": "YOUR_KEY"}
)
print(response.json())
Response:
{
"results": [
{"field_name": "email", "valid": true, "detail": "Valid email format"},
{"field_name": "mobile", "valid": true, "detail": "Valid GB mobile number"},
{"field_name": "bank", "valid": true, "detail": "Valid IBAN — country code GB"},
{"field_name": "card", "valid": true, "detail": "Valid card number — Visa"},
{"field_name": "website", "valid": true, "detail": "Valid URL using https"}
],
"summary": {"total": 5, "valid": 5, "invalid": 0},
"processing_ms": 3.42
}
One call. Five fields validated. Summary included.
The 13 supported formats
email, phone (200+ countries), IBAN, EU VAT, credit card
(with Luhn checksum), UK postcode, US ZIP, URL, UUID,
IPv4/IPv6, SSN, EIN, ISO dates.
How I built it
Stack: Python, FastAPI, Railway, RapidAPI
Libraries that did the heavy lifting:
python-stdnum— handles IBAN, VAT, SSN, EIN with
proper checksum validation
phonenumbers— Google's library, handles 200+ countries
diffprivlib— not used here but I've used it in other projects
slowapi— rate limiting in one line on top of FastAPI
The core is a validator registry — a dict that maps format
type strings to validator functions:
VALIDATORS = {
"email": validate_email,
"phone": validate_phone,
"iban": validate_iban,
"credit_card": validate_credit_card,
# ... 9 more
}
def validate(format_type: str, value: str) -> dict:
fn = VALIDATORS.get(format_type.lower())
if fn is None:
return {"valid": False, "detail": f"Unknown format '{format_type}'"}
return fn(value)
The batch endpoint just loops over the fields array and calls
this for each one. The whole thing is under 200 lines of code.
Deployment: pushed to GitHub, connected to Railway,
auto-deployed. Live in about 90 seconds.
Listing: added to RapidAPI with a free tier (100 req/day),
Basic ($9/mo), and Pro ($29/mo).
Total time from idea to live API: one day.
Total cost: $0.
Why batch matters
Most validation APIs are single-format. You need a different
endpoint — often a different provider — for each field type.
That means:
- Multiple API keys to manage
- Multiple rate limits to track
- Multiple failure points in your pipeline
- More latency from sequential calls
MultiValidator sends everything in one request and returns
a summary with valid/invalid counts. Your form backend makes
one call and knows exactly which fields to reject.
What I learned
This was my first time shipping a paid API product. A few things
stood out:
FastAPI is excellent for this. Auto-generated docs at /docs,
Pydantic validation, rate limiting with slowapi — everything
just works.
python-stdnum is underrated. It handles IBAN checksum
validation, EU VAT formats across all countries, SSN invalid
range detection — all out of the box. I expected to write a lot
of regex. I barely wrote any.
Railway is genuinely zero-friction deployment. Connect GitHub
repo, done. Auto-deploys on every push.
The hardest part wasn't the code. It was deciding what
formats to include and what the response schema should look like.
API design decisions feel small but they're what users actually
interact with.
Try it
The API is live on RapidAPI with a free tier — 100 requests/day,
no credit card required.
Search MultiValidator Batch Format Validation on RapidAPI
or find it at:
rapidapi.com/rijul.ssh/api/multivalidator-batch-format-validation
Happy to answer questions about the build in the comments.
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