Base64 Encoded String Regex for PHP

What this pattern does

This page provides a well-structured, multi-part regular expression for matching base64 encoded string, 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
// Base64 Encoded String
// ReDoS-safe | RegexVault — Web & Network > Misc

define('BASE64_ENCODED_STRING_PATTERN', '/^(?:[A-Za-z0-9+\/]{4})*(?:[A-Za-z0-9+\/]{2}==|[A-Za-z0-9+\/]{3}=)?$/');

function validate_base64_encoded_string(string $input): bool {
    return (bool) preg_match(BASE64_ENCODED_STRING_PATTERN, $input);
}

// Example
var_dump(validate_base64_encoded_string("SGVsbG8sIFdvcmxkIQ==")); // bool(true)

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > Misc 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

Empty string is valid base64 (encodes empty byte string). This pattern validates standard base64 — for URL-safe base64, replace + with \- and / with _.