IPv6 with CIDR Prefix Length Regex for Python

What this pattern does

This page provides a well-structured, multi-part regular expression for matching ipv6 with cidr prefix length, ported and verified for Python. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. The snippet below is ready to drop into your Python project — whether you're validating in a Django view, a FastAPI endpoint, or a standalone data processing script.

Python Implementation

# IPv6 with CIDR Prefix Length
# ReDoS-safe | RegexVault — Web & Network > IPv6

import re

ipv6_with_cidr_prefix_length_pattern = re.compile(r'^[0-9a-fA-F:]+\/(?:12[0-8]|1[01][0-9]|[1-9][0-9]|[0-9])$')

def validate_ipv6_with_cidr_prefix_length(value: str) -> bool:
    return bool(ipv6_with_cidr_prefix_length_pattern.fullmatch(value))

# Example
print(validate_ipv6_with_cidr_prefix_length("2001:db8::/32"))  # True

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > IPv6 category and carries a ReDoS-safe certification. That matters for Python developers because particularly important in Python web servers where CPU-bound regex operations can stall concurrent request handling. 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 IPv6 portion uses a simplified [0-9a-fA-F:]+ to avoid duplicating the full IPv6 validation. Pair with net-ipv6-02 for full address validation before applying this pattern.