BCP 47 / IETF Language Tag Regex for Python

What this pattern does

This page provides a well-structured, multi-part regular expression for matching bcp 47 / ietf language tag, 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

# BCP 47 / IETF Language Tag
# ReDoS-safe | RegexVault — Localization > Locale & Language

import re

bcp_47_ietf_language_tag_pattern = re.compile(r'^[a-zA-Z]{2,3}(?:-[a-zA-Z]{4})?(?:-(?:[A-Z]{2}|[0-9]{3}))?$')

def validate_bcp_47_ietf_language_tag(value: str) -> bool:
    return bool(bcp_47_ietf_language_tag_pattern.fullmatch(value))

# Example
print(validate_bcp_47_ietf_language_tag("en"))  # True

Test Cases

When to use this pattern

This pattern is drawn from the Localization > Locale & Language 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

Language subtag examples: en=English, zh=Chinese, es=Spanish. Script subtags: Hant=Traditional Chinese, Hans=Simplified Chinese, Latn=Latin. Region subtags: US, CN, TW (ISO 3166-1 alpha-2). Use ISO 639-1 (2-char) or ISO 639-2 (3-char) codes.