Domain Name Maximum Length (253 chars) Regex for Python

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching domain name maximum length (253 chars), 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

# Domain Name Maximum Length (253 chars)
# ReDoS-safe | RegexVault — Web & Network > Domain

import re

domain_name_maximum_length_253_chars_pattern = re.compile(r'^(?=.{1,253}$)(?:[a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?\.)*[a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?$')

def validate_domain_name_maximum_length_253_chars(value: str) -> bool:
    return bool(domain_name_maximum_length_253_chars_pattern.fullmatch(value))

# Example
print(validate_domain_name_maximum_length_253_chars("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 lookahead (?=.{1,253}$) enforces the 253-character DNS limit. This is especially important for domains constructed programmatically. Go's RE2 does not support lookaheads — perform the length check separately in Go.