How do I validate Codice Fiscale + P.IVA at Magento checkout?

Two Italian tax identifiers, two different validators, both must be checked at checkout, not after, or you’ll get SDI rejections downstream.

• Codice Fiscale, 16 alphanumeric chars, encodes surname / name / DOB / sex / municipality. Validated by checksum + format regex (no online API needed for the algorithm; AdE has a verification endpoint for sanity).
• Partita IVA (P.IVA), 11 digits. Validated by Luhn-like checksum + via AdE’s P.IVA lookup (free service) for active status. For EU intra-Community B2B: VIES validation of the IT-prefixed P.IVA confirms eligibility for 0% reverse-charge IVA.

Magento implementation:

1. Add Codice Fiscale + P.IVA fields to customer and quote_address entities.
2. Validate format + checksum client-side (instant feedback) and server-side (authoritative).
3. For B2B: call VIES on save; cache 30 days; apply reverse-charge IVA (0%) if valid.
4. Block checkout submission with invalid IDs, never let bad data hit SDI.
5. Pre-fill billing address with the AdE-returned company name + address.