RFC

The Registro Federal de Contribuyentes, the tax ID the SAT gives people and companies.

Validate and parse

import { checkRfc } from "mexico-utils";

checkRfc("EKU9003173C9");
// {
//   ok: true,
//   value: {
//     rfc: "EKU9003173C9",
//     type: "company",
//     date: "1990-03-17",
//     homoclave: "3C",
//     checkDigitMatches: true
//   }
// }

date is the birth date for people and the incorporation date for companies.

The check digit

The last character is computed with SAT's algorithm and reported as checkDigitMatches. It doesn't make an RFC invalid unless you ask for it:

checkRfc("CACX7605101P9");
// { ok: true, value: { ..., checkDigitMatches: false } }

checkRfc("CACX7605101P9", { checkDigit: true });
// { ok: false, error: "check-digit" }

Why: some real RFCs, mostly old ones, don't match their own check digit. Turning someone away because SAT made a mistake decades ago is worse than letting the RFC through. If you only deal with new RFCs, turn it on.