ICU Locale Identifier Regex for PHP

What this pattern does

This page provides a well-structured, multi-part regular expression for matching icu locale identifier, ported and verified for PHP. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. The snippet below is ready to drop into your PHP project — whether you're validating in a Laravel validator, a WordPress plugin, or a standalone PHP script.

Php Implementation

<?php
// ICU Locale Identifier
// ReDoS-safe | RegexVault — Localization > Locale & Language

define('ICU_LOCALE_IDENTIFIER_PATTERN', '/^[a-z]{2,3}(?:_(?:[A-Z]{2}|[0-9]{3})(?:_(?:[A-Z0-9]{2,8}))?)?$/');

function validate_icu_locale_identifier(string $input): bool {
    return (bool) preg_match(ICU_LOCALE_IDENTIFIER_PATTERN, $input);
}

// Example
var_dump(validate_icu_locale_identifier("en")); // bool(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 PHP developers because especially relevant in PHP where PCRE backtracking limits can trigger silent failures on malicious input. 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).