Fully Qualified Domain Name (FQDN) Regex for JavaScript

What this pattern does

This page provides a well-structured, multi-part regular expression for matching fully qualified domain name (fqdn), 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

// Fully Qualified Domain Name (FQDN)
// ReDoS-safe | RegexVault — Web & Network > Domain

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

function validateFullyQualifiedDomainNameFqdn(input: string): boolean {
  return fullyQualifiedDomainNameFqdnRegex.test(input);
}

// Example
console.log(validateFullyQualifiedDomainNameFqdn("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

The trailing dot is the canonical FQDN form. DNS software accepts FQDNs natively. Strip trailing dots before use in HTTP Host headers — browsers do not send the trailing dot.