Accept-Language Header Value Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching accept-language header value, 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

// Accept-Language Header Value
// ReDoS-safe | RegexVault — Web & Network > HTTP

const acceptlanguageHeaderValueRegex = /^[a-zA-Z]{1,8}(?:-[a-zA-Z0-9]{1,8})*(?:;q=(?:0(?:\.[0-9]{1,3})?|1(?:\.0{1,3})?))?(?:\s*,\s*[a-zA-Z]{1,8}(?:-[a-zA-Z0-9]{1,8})*(?:;q=(?:0(?:\.[0-9]{1,3})?|1(?:\.0{1,3})?))?)*$/i;

function validateAcceptlanguageHeaderValue(input: string): boolean {
  return acceptlanguageHeaderValueRegex.test(input);
}

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

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > HTTP 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

Quality factor (q) must be between 0 and 1 with up to 3 decimal places. Default q value is 1.0 when omitted. Used for content negotiation.