MD5 Hash (Deprecated — Detection Only) Regex for PHP

What this pattern does

This page provides a lightweight, single-purpose regular expression for matching md5 hash (deprecated — detection only), ported and verified for PHP. In security-sensitive code, using an unverified regex can open the door to both false positives and denial-of-service attacks. 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
// MD5 Hash (Deprecated — Detection Only)
// ReDoS-safe | RegexVault — Security > Password Formats

define('MD5_HASH_DEPRECATED_DETECTION_ONLY_PATTERN', '/^[a-f0-9]{32}$/');

function validate_md5_hash_deprecated_detection_only(string $input): bool {
    return (bool) preg_match(MD5_HASH_DEPRECATED_DETECTION_ONLY_PATTERN, $input);
}

// Example
var_dump(validate_md5_hash_deprecated_detection_only("d41d8cd98f00b204e9800998ecf8427e")); // bool(true)

Test Cases

When to use this pattern

This pattern is drawn from the Security > Password Formats 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

MD5 is cryptographically broken — collision attacks are trivial, preimage attacks are feasible. Never use MD5 for password hashing or security purposes. Use only for checksums where collision resistance is not required (e.g., non-security file deduplication). Include this pattern only for detection/migration purposes.