Fully Qualified Domain Name (FQDN) Regex for Python

What this pattern does

This page provides a well-structured, multi-part regular expression for matching fully qualified domain name (fqdn), 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

# Fully Qualified Domain Name (FQDN)
# ReDoS-safe | RegexVault — Web & Network > Domain

import re

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

def validate_fully_qualified_domain_name_fqdn(value: str) -> bool:
    return bool(fully_qualified_domain_name_fqdn_pattern.fullmatch(value))

# Example
print(validate_fully_qualified_domain_name_fqdn("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

The trailing dot is the canonical FQDN form. DNS software accepts FQDNs natively. Strip trailing dots before use in HTTP Host headers — browsers do not send the trailing dot.