Accept-Language Header Value Regex for PHP

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching accept-language header value, 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
// Accept-Language Header Value
// ReDoS-safe | RegexVault — Web & Network > HTTP

define('ACCEPTLANGUAGE_HEADER_VALUE_PATTERN', '/^[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})?))?)*$/');

function validate_acceptlanguage_header_value(string $input): bool {
    return (bool) preg_match(ACCEPTLANGUAGE_HEADER_VALUE_PATTERN, $input);
}

// Example
var_dump(validate_acceptlanguage_header_value("en")); // bool(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 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

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.