Subdomain Extraction Regex for Python

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching subdomain extraction, 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

# Subdomain Extraction
# ReDoS-safe | RegexVault — Web & Network > Domain

import re

subdomain_extraction_pattern = re.compile(r'^(?:([a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?\.)+)?[a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z]{2,63})$')

def validate_subdomain_extraction(value: str) -> bool:
    return bool(subdomain_extraction_pattern.fullmatch(value))

# Example
print(validate_subdomain_extraction("www.example.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

Capture group 1 contains the subdomain prefix (with trailing dot) if present, empty/null if it's a bare SLD+TLD. PSL (Public Suffix List) is needed for accurate eTLD+1 extraction.