IPv4 Loopback Address Regex for Java

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching ipv4 loopback address, ported and verified for Java. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. The snippet below is ready to drop into your Java project — whether you're validating in a Spring Boot controller, a Jakarta EE service, or a standalone utility class.

Java Implementation

// IPv4 Loopback Address
// ReDoS-safe | RegexVault — Web & Network > IPv4

import java.util.regex.Pattern;

public class Ipv4LoopbackAddressValidator {
    private static final Pattern PATTERN =
        Pattern.compile("^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])$");

    public static boolean validate(String input) {
        return PATTERN.matcher(input).matches();
    }

    // Example
    public static void main(String[] args) {
        System.out.println(validate("127.0.0.1")); // 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 Java developers because critical in Java applications since the JVM regex engine uses backtracking and is susceptible to ReDoS without careful pattern design. 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.