CIDR Notation (IPv6) Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching cidr notation (ipv6), 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

// CIDR Notation (IPv6)
// ReDoS-safe | RegexVault — Security > Network Security

const cidrNotationIpv6Regex = /^(?:[0-9a-fA-F]{1,4}:){7}[0-9a-fA-F]{1,4}\\/(?:1[01][0-9]|12[0-8]|[0-9]{1,2})$|^::(?:[fF]{4}(?::0{1,4})?:)?(?:25[0-5]|2[0-4][0-9]|[01]?[0-9]{1,2})(?:\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9]{1,2})){3}\\/(?:1[01][0-9]|12[0-8]|[0-9]{1,2})$|^(?:[0-9a-fA-F]{0,4}:){1,7}(?::[0-9a-fA-F]{0,4}){0,7}\\/(?:1[01][0-9]|12[0-8]|[0-9]{1,2})$/i;

function validateCidrNotationIpv6(input: string): boolean {
  return cidrNotationIpv6Regex.test(input);
}

// Example
console.log(validateCidrNotationIpv6("2001:db8::/32")); // 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

IPv6 CIDR prefixes: /48 is common for ISP assignments, /64 for subnets (standard for SLAAC), /128 for single hosts. ::/0 is the IPv6 default route. fe80::/10 is link-local. ::1/128 is loopback.