Great topic! Chilean RUT validation is a great case study for region-specific ID systems. Here's a practical breakdown:

RUT Format & Validation Logic A RUT looks like 12.345.678-9 where the last character is the check digit (0-9 or K). The validation algorithm uses modulo 11:

function validateRUT(rut) {
  const clean = rut.replace(/[^0-9kK]/g, '');
  if (clean.length < 2) return false;
  const body = clean.slice(0, -1);
  const dv = clean.slice(-1).toUpperCase();
  let sum = 0, multiplier = 2;
  for (let i = body.length - 1; i >= 0; i--) {
    sum += parseInt(body[i]) * multiplier;
    multiplier = multiplier === 7 ? 2 : multiplier + 1;
  }
  const expected = 11 - (sum % 11);
  const dvExpected = expected === 11 ? '0' : expected === 10 ? 'K' : String(expected);
  return dv === dvExpected;
}

Libraries to consider:

• rutjs (npm) — lightweight, handles formatting + validation
• @fdograph/rut-utilities — more complete, TypeScript support
• vue-rut if you're on Vue.js

Best Practices for Scalable Apps:

1. Normalize on input — strip dots/dashes before storing, only format for display
2. Index the clean version in your DB for fast lookups
3. Client-side + server-side validation — never trust only the frontend
4. For other LATAM countries: Argentina uses CUIL/CUIT (similar mod11), Brazil uses CPF (mod11 with different weights), Colombia uses NIT. Consider a library like laralib/l5-brasil or building a shared validation service if you're multi-country.

Happy to share more on the multi-country architecture if that's relevant to your project!