Golang : Use regular expression to validate domain name

There are times when a user enter an invalid domain name as input and it is good to validate the domain name before processing further or store into database. So far, the best way to validate a domain name in Golang is still via regular expression.

There are couple of regular expressions out there that are generally accepted regular expression to validate a domain name. However, for this tutorial we will use :

^(([a-zA-Z]{1})|([a-zA-Z]{1}[a-zA-Z]{1})|([a-zA-Z]{1}[0-9]{1})|([0-9]{1}[a-zA-Z]{1})|

([a-zA-Z0-9][a-zA-Z0-9-_]{1,61}[a-zA-Z0-9])).([a-zA-Z]{2,6}|[a-zA-Z0-9-]{2,30}.[a-zA-Z]{2,3})$

Here we go!

 package main

 import (
 "fmt"
 "regexp"
 )

 func validateDomainName(domain string) bool {

 // Golang does not support Perl syntax ((?

 // will throw out :
 // error parsing regexp: invalid or unsupported Perl syntax: `(?!`

 //patternStr := "^((?!-)[A-Za-z0-9-]{1,63}(?<!-)\\.)+[A-Za-z]{2,6}$"

 // use regular expression without Perl syntax

 RegExp := regexp.MustCompile(`^(([a-zA-Z]{1})|([a-zA-Z]{1}[a-zA-Z]{1})|([a-zA-Z]{1}[0-9]{1})|([0-9]{1}[a-zA-Z]{1})|([a-zA-Z0-9][a-zA-Z0-9-_]{1,61}[a-zA-Z0-9]))\.([a-zA-Z]{2,6}|[a-zA-Z0-9-]{2,30}\.[a-zA-Z
 ]{2,3})$`)

 return RegExp.MatchString(domain)
 }

 func main() {

 domName := "www.golang.org"

 if !validateDomainName(domName) {
 fmt.Printf("Domain Name %s is invalid\n", domName)
 } else {
 fmt.Printf("Domain Name %s is VALID\n", domName)
 }

 domName = "www.socketloop,.com"

 if !validateDomainName(domName) {
 fmt.Printf("Domain Name %s is invalid\n", domName)
 } else {
 fmt.Printf("Domain Name %s is VALID\n", domName)
 }

 domName = "subdomain-socketloop.com"

 if !validateDomainName(domName) {
 fmt.Printf("Domain Name %s is invalid\n", domName)
 } else {
 fmt.Printf("Domain Name %s is VALID\n", domName)
 }

 domName = "-socketloop.com" // invalid starts with hyphen

 if !validateDomainName(domName) {
 fmt.Printf("Domain Name %s is invalid\n", domName)
 } else {
 fmt.Printf("Domain Name %s is VALID\n", domName)
 }

 domName = "socketloop.co_" // invalid ends with underscore

 if !validateDomainName(domName) {
 fmt.Printf("Domain Name %s is invalid\n", domName)
 } else {
 fmt.Printf("Domain Name %s is VALID\n", domName)
 }

 domName = "subdomain.socketloop.com"

 if !validateDomainName(domName) {
 fmt.Printf("Domain Name %s is invalid\n", domName)
 } else {
 fmt.Printf("Domain Name %s is VALID\n", domName)
 }

 }

Output :

Domain Name www.golang.org is VALID

Domain Name www.socketloop,.com is invalid

Domain Name subdomain-socketloop.com is VALID

Domain Name -socketloop.com is invalid

Domain Name socketloop.co_ is invalid

Domain Name subdomain.socketloop.com is VALID

References :

https://www.debuggex.com/r/y4Xe_hDVO11bv1DV

http://stackoverflow.com/questions/10306690/domain-name-validation-with-regex