Subdomain Extraction Regex for Go

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching subdomain extraction, 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

// Subdomain Extraction
// ReDoS-safe | RegexVault — Web & Network > Domain

package validation

import "regexp"

var subdomainExtractionRe = regexp.MustCompile(`^(?:([a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?\.)+)?[a-zA-Z0-9](?:[a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z]{2,63})$`)

func ValidateSubdomainExtraction(s string) bool {
    return subdomainExtractionRe.MatchString(s)
}

// Example
// fmt.Println(ValidateSubdomainExtraction("www.example.com")) // true

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > Domain 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

Capture group 1 contains the subdomain prefix (with trailing dot) if present, empty/null if it's a bare SLD+TLD. PSL (Public Suffix List) is needed for accurate eTLD+1 extraction.