X.509 Certificate Serial Number Regex for PHP

What this pattern does

This page provides a well-structured, multi-part regular expression for matching x.509 certificate serial number, 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
// X.509 Certificate Serial Number
// ReDoS-safe | RegexVault — Security > Certificates & PKI

define('X509_CERTIFICATE_SERIAL_NUMBER_PATTERN', '/^[0-9a-f]{2}(?::[0-9a-f]{2}){7,19}$/');

function validate_x509_certificate_serial_number(string $input): bool {
    return (bool) preg_match(X509_CERTIFICATE_SERIAL_NUMBER_PATTERN, $input);
}

// Example
var_dump(validate_x509_certificate_serial_number("01:23:45:67:89:ab:cd:ef:01:23:45:67:89:ab:cd:ef")); // bool(true)

Test Cases

When to use this pattern

This pattern is drawn from the Security > Certificates & PKI 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

X.509 serial numbers are 1-20 byte (8-160 bit) integers, typically displayed in colon-separated hex pairs. CA/Browser Forum requires serial numbers be at least 64 bits and generated with at least 64 bits of entropy. Used for certificate revocation lookups (CRL, OCSP).