Fully Qualified Domain Name (FQDN) Regex for Go

What this pattern does

This page provides a well-structured, multi-part regular expression for matching fully qualified domain name (fqdn), 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

// Fully Qualified Domain Name (FQDN)
// ReDoS-safe | RegexVault — Web & Network > Domain

package validation

import "regexp"

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

func ValidateFullyQualifiedDomainNameFqdn(s string) bool {
    return fullyQualifiedDomainNameFqdnRe.MatchString(s)
}

// Example
// fmt.Println(ValidateFullyQualifiedDomainNameFqdn("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

The trailing dot is the canonical FQDN form. DNS software accepts FQDNs natively. Strip trailing dots before use in HTTP Host headers — browsers do not send the trailing dot.