ICU Locale Identifier Regex for JavaScript

What this pattern does

This page provides a well-structured, multi-part regular expression for matching icu locale identifier, 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

// ICU Locale Identifier
// ReDoS-safe | RegexVault — Localization > Locale & Language

const icuLocaleIdentifierRegex = /^[a-z]{2,3}(?:_(?:[A-Z]{2}|[0-9]{3})(?:_(?:[A-Z0-9]{2,8}))?)?$/;

function validateIcuLocaleIdentifier(input: string): boolean {
  return icuLocaleIdentifierRegex.test(input);
}

// Example
console.log(validateIcuLocaleIdentifier("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

ICU uses underscores as separators (en_US) while BCP 47 uses hyphens (en-US). Java's Locale class uses underscore format. Unicode CLDR supports both via normalization. es_419 is Spanish for Latin America (UN area code 419).