Accept-Language Header Value Regex for Python

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching accept-language header value, 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

# Accept-Language Header Value
# ReDoS-safe | RegexVault — Web & Network > HTTP

import re

acceptlanguage_header_value_pattern = re.compile(r'^[a-zA-Z]{1,8}(?:-[a-zA-Z0-9]{1,8})*(?:;q=(?:0(?:\.[0-9]{1,3})?|1(?:\.0{1,3})?))?(?:\s*,\s*[a-zA-Z]{1,8}(?:-[a-zA-Z0-9]{1,8})*(?:;q=(?:0(?:\.[0-9]{1,3})?|1(?:\.0{1,3})?))?)*$')

def validate_acceptlanguage_header_value(value: str) -> bool:
    return bool(acceptlanguage_header_value_pattern.fullmatch(value))

# Example
print(validate_acceptlanguage_header_value("en"))  # True

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > HTTP 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

Quality factor (q) must be between 0 and 1 with up to 3 decimal places. Default q value is 1.0 when omitted. Used for content negotiation.