IPv4 Loopback Address Regex for PHP

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching ipv4 loopback address, 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
// IPv4 Loopback Address
// ReDoS-safe | RegexVault — Web & Network > IPv4

define('IPV4_LOOPBACK_ADDRESS_PATTERN', '/^127\.(?:(?:25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9][0-9]|[0-9])\.){2}(?:25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9][0-9]|[0-9])$/');

function validate_ipv4_loopback_address(string $input): bool {
    return (bool) preg_match(IPV4_LOOPBACK_ADDRESS_PATTERN, $input);
}

// Example
var_dump(validate_ipv4_loopback_address("127.0.0.1")); // bool(true)

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > IPv4 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

The entire 127.0.0.0/8 block is reserved for loopback. 127.0.0.1 is the default but any address in the block routes to the local machine on Linux.