Parse a hostname into a slice from a URL https://play.golang.com/p/et2gNy1MgjD

et2gNy1MgjD.go

package main

import (
    "fmt"
    "log"
    "net/url"
    "strings"
)

func ParseHostname(domain string) []string {
    u, err := url.Parse(domain)
    if err != nil {
        log.Fatal(err)
    }
    hostname := u.Hostname()
    hostname_slice := strings.Split(hostname, ".")
    return hostname_slice
}

func main() {
    parts := ParseHostname("http://subdomain.example.com:3000")
    fmt.Println(parts)
}

et2gNy1MgjD_test.go

package main

import (
    "reflect"
    "testing"
)

func TestParseHostname(t *testing.T) {

    t.Run("test subdomain.example.com", func(t *testing.T) {
        got := ParseHostname("http://subdomain.example.com")
        want := []string{"subdomain", "example", "com"}
        
        if !reflect.DeepEqual(got, want) {
            t.Errorf("got %v want %v", got, want)
        }
    })
}