Subdomain Extraction Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching subdomain extraction, 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

// Subdomain Extraction
// ReDoS-safe | RegexVault — Web & Network > Domain

const subdomainExtractionRegex = /^(?:([a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?\.)+)?[a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z]{2,63})$/i;

function validateSubdomainExtraction(input: string): boolean {
  return subdomainExtractionRegex.test(input);
}

// Example
console.log(validateSubdomainExtraction("www.example.com")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > Domain 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

Capture group 1 contains the subdomain prefix (with trailing dot) if present, empty/null if it's a bare SLD+TLD. PSL (Public Suffix List) is needed for accurate eTLD+1 extraction.