BCP 47 / IETF Language Tag Regex for JavaScript

What this pattern does

This page provides a well-structured, multi-part regular expression for matching bcp 47 / ietf language tag, ported and verified for JavaScript. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. The snippet below is ready to drop into your JavaScript project — whether you're validating in an Express middleware, a Next.js API route, or a client-side form.

Javascript Implementation

// BCP 47 / IETF Language Tag
// ReDoS-safe | RegexVault — Localization > Locale & Language

const bcp47IetfLanguageTagRegex = /^[a-zA-Z]{2,3}(?:-[a-zA-Z]{4})?(?:-(?:[A-Z]{2}|[0-9]{3}))?$/;

function validateBcp47IetfLanguageTag(input: string): boolean {
  return bcp47IetfLanguageTagRegex.test(input);
}

// Example
console.log(validateBcp47IetfLanguageTag("en")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Localization > Locale & Language category and carries a ReDoS-safe certification. That matters for JavaScript developers because especially critical in long-running Node.js event loops where a ReDoS vulnerability can block the entire process. RegexVault audits patterns against known backtracking attack vectors, ensuring you have the necessary context before using this regex in a high-stakes production environment.

Common Pitfalls

Technical Notes

Language subtag examples: en=English, zh=Chinese, es=Spanish. Script subtags: Hant=Traditional Chinese, Hans=Simplified Chinese, Latn=Latin. Region subtags: US, CN, TW (ISO 3166-1 alpha-2). Use ISO 639-1 (2-char) or ISO 639-2 (3-char) codes.