Punycode / Internationalized Domain Name Regex for Python

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching punycode / internationalized domain name, 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

# Punycode / Internationalized Domain Name
# ReDoS-safe | RegexVault — Web & Network > Domain

import re

punycode_internationalized_domain_name_pattern = re.compile(r'^(?:(?:xn--[a-zA-Z0-9]{1,59}|[a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?)?\.)*(?:xn--[a-zA-Z0-9]{1,59}|[a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?)$')

def validate_punycode_internationalized_domain_name(value: str) -> bool:
    return bool(punycode_internationalized_domain_name_pattern.fullmatch(value))

# Example
print(validate_punycode_internationalized_domain_name("xn--nxasmq6b.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 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

IDN TLDs also use Punycode (e.g., .xn--p1ai for .рф). Always normalize and validate IDN using a dedicated library (e.g., idna in Python) rather than relying solely on regex.