Domain Label (Single Segment) Regex for Python

What this pattern does

This page provides a well-structured, multi-part regular expression for matching domain label (single segment), 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 Label (Single Segment)
# ReDoS-safe | RegexVault — Web & Network > Domain

import re

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

def validate_domain_label_single_segment(value: str) -> bool:
    return bool(domain_label_single_segment_pattern.fullmatch(value))

# Example
print(validate_domain_label_single_segment("example"))  # 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 63-character maximum is per RFC 1034. The last invalid test case is 64 chars (one over the limit). Use for validating individual hostname components.