IPv4 Loopback Address Regex for Go

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching ipv4 loopback address, ported and verified for Go. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. The snippet below is ready to drop into your Go project — whether you're validating in a Gin handler, a gRPC service, or a command-line tool.

Go Implementation

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

package validation

import "regexp"

var ipv4LoopbackAddressRe = regexp.MustCompile(`^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])$`)

func ValidateIpv4LoopbackAddress(s string) bool {
    return ipv4LoopbackAddressRe.MatchString(s)
}

// Example
// fmt.Println(ValidateIpv4LoopbackAddress("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 Go developers because Go's RE2 engine is inherently safe from catastrophic backtracking, but this pattern has been additionally verified for correctness. 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.