Private / Reserved IPv4 Ranges Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching private / reserved ipv4 ranges, ported and verified for JavaScript. In security-sensitive code, using an unverified regex can open the door to both false positives and denial-of-service attacks. 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

// Private / Reserved IPv4 Ranges
// ReDoS-safe | RegexVault — Security > Network Security

const privateReservedIpv4RangesRegex = /^(?:10\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)|172\.(?:1[6-9]|2[0-9]|3[01])\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)|192\.168\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)|127\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)|169\.254\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?))$/;

function validatePrivateReservedIpv4Ranges(input: string): boolean {
  return privateReservedIpv4RangesRegex.test(input);
}

// Example
console.log(validatePrivateReservedIpv4Ranges("10.0.0.1")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Security > Network Security 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

Private ranges: 10.0.0.0/8 (Class A, RFC 1918), 172.16.0.0/12 (Class B, RFC 1918), 192.168.0.0/16 (Class C, RFC 1918), 127.0.0.0/8 (loopback), 169.254.0.0/16 (link-local APIPA). Use for SSRF protection: reject private IPs in user-supplied URLs.