ICU Locale Identifier Regex for Python

What this pattern does

This page provides a well-structured, multi-part regular expression for matching icu locale identifier, 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

# ICU Locale Identifier
# ReDoS-safe | RegexVault — Localization > Locale & Language

import re

icu_locale_identifier_pattern = re.compile(r'^[a-z]{2,3}(?:_(?:[A-Z]{2}|[0-9]{3})(?:_(?:[A-Z0-9]{2,8}))?)?$')

def validate_icu_locale_identifier(value: str) -> bool:
    return bool(icu_locale_identifier_pattern.fullmatch(value))

# Example
print(validate_icu_locale_identifier("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

ICU uses underscores as separators (en_US) while BCP 47 uses hyphens (en-US). Java's Locale class uses underscore format. Unicode CLDR supports both via normalization. es_419 is Spanish for Latin America (UN area code 419).