CIDR Notation (IPv4) Regex for Python

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching cidr notation (ipv4), ported and verified for Python. 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 Python project — whether you're validating in a Django view, a FastAPI endpoint, or a standalone data processing script.

Python Implementation

# CIDR Notation (IPv4)
# ReDoS-safe | RegexVault — Security > Network Security

import re

cidr_notation_ipv4_pattern = re.compile(r'^(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\/(?:[0-9]|[12][0-9]|3[0-2])$')

def validate_cidr_notation_ipv4(value: str) -> bool:
    return bool(cidr_notation_ipv4_pattern.fullmatch(value))

# Example
print(validate_cidr_notation_ipv4("192.168.1.0/24"))  # 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 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

/0 = all IPs (default route), /32 = single host. Subnets should have a host part of all zeros for canonical CIDR notation (192.168.1.0/24, not 192.168.1.5/24). Common in firewall rules, security groups, and IP allowlists.