ISO 3166-1 Alpha-2 Country Code Regex for PHP

What this pattern does

This page provides a lightweight, single-purpose regular expression for matching iso 3166-1 alpha-2 country code, 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
// ISO 3166-1 Alpha-2 Country Code
// ReDoS-safe | RegexVault — Localization > Locale & Language

define('ISO_31661_ALPHA2_COUNTRY_CODE_PATTERN', '/^[A-Z]{2}$/');

function validate_iso_31661_alpha2_country_code(string $input): bool {
    return (bool) preg_match(ISO_31661_ALPHA2_COUNTRY_CODE_PATTERN, $input);
}

// Example
var_dump(validate_iso_31661_alpha2_country_code("US")); // 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

ISO 3166-1 alpha-2 uses uppercase. Exceptions: UK is not a valid code (GB is used for the United Kingdom). XK is used for Kosovo informally. EU is a special supranational code. Validate against the official ISO 3166 Maintenance Agency list.