validate a text input for BCP 47 language tag

by Imri Paloja

JavaScript

const bcp47Regex =
  /^[a-zA-Z]{2,3}(?:-[a-zA-Z]{4})?(?:-[a-zA-Z]{2}|\-[0-9]{3})?(?:-[a-zA-Z0-9]{5,8})*$/;

function isValidBCP47(tag) {
  return bcp47Regex.test(tag);
}

console.log(isValidBCP47('en')); // true
console.log(isValidBCP47('en-US')); // true
console.log(isValidBCP47('zh-Hans-CN')); // true
console.log(isValidBCP47('english')); // false

function normalizeBCP47(tag) {
  return tag
    .split('-')
    .map((part, i) => {
      if (i === 0) return part.toLowerCase(); // language
      if (part.length === 4)
        // script
        return part[0].toUpperCase() + part.slice(1).toLowerCase();
      if (part.length === 2 || part.length === 3)
        // region
        return part.toUpperCase();
      return part.toLowerCase(); // variants
    })
    .join('-');
}

console.log(normalizeBCP47('EN-hans-us')); // en-Hans-US