finding, parsing and validating email addresses

 import "fmt"

func main() {
    email := "learning@gmail.com"
    atTrue := false
    s := ""
    for i := 0; i < len(email); i++ {
        if atTrue {
            s += string(email[i])
        }
        if string(email[i]) == "@" {
            atTrue = true
        }
    }
    fmt.Println(s)
}

current output: gmail.com

expect output: Domain: gmail and TLD: com

How to indicate looping from certain string to certain string?

Use strings.Split function

	email := "learning@gmail.com"
split := strings.Split(email, "@")
split = strings.Split(split[1], ".")
fmt.Println("Domain:", split[0])
fmt.Println("TLD:", split[1])

optionally you can validate an email string with mail.ParseAddress function

	email := "learning@gmail.com"
addr, err := mail.ParseAddress(email)
if err != nil {
log.Fatal(err)
}

> what if the TLD goes like this co.id, does the output still co.id ? –
@zacharyy

simply try to find the first index of . (strings.Index) and use one to separate the string

	email := "learning@gmail.co.id"
split := strings.Split(email, "@")
index := strings.Index(split[1], ".")
fmt.Println("Domain:", split[1][:index])
fmt.Println("TLD:", split[1][index+1:])

or use regexp.Split function

	email := "learning@gmail.co.id"
split := strings.Split(email, "@")
split = regexp.MustCompile("[.]").
Split(split[1], 2)
fmt.Println("Domain:", split[0])
fmt.Println("TLD:", split[1])

or strings.SplitN

    email := "learning@gmail.co.id"
split := strings.Split(email, "@")
split = strings.SplitN(split[1], ".", 2)
fmt.Println("Domain:", split[0])
fmt.Println("TLD:", split[1])

or strings.Cut

	_, host, found := strings.Cut(email, "@")
if !found {
t.Fatal("fail cut to host (Domain + TLD)")
}
domain, tld, found := strings.Cut(host, ".")
if !found {
t.Fatal("fail cut to Domain and TLD")
}
fmt.Println("Domain:", domain)
fmt.Println("TLD:", tld)

👉🏻 <kbd>PLAYGROUND</kbd> 👈🏻

Benchmarks

	const (
quantity   = 10000
userLength = 10
domain     = "gmail.com"
)
var (
letterRunes = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
emails      = make([]string, 0, quantity)
seeded      = rand.New(rand.NewSource(time.Now().UnixMilli()))
)
for i := 0; i < quantity; i++ {
user := make([]rune, userLength)
for i := range user {
user[i] = letterRunes[seeded.Intn(len(letterRunes))]
}
emails = append(emails, fmt.Sprintf("%s@%s", string(user), domain))
}
b.Run("strings.Split", func(b *testing.B) {
b.ReportAllocs()
b.ResetTimer()
var domain, tld string
for i := 0; i < b.N; i++ {
for _, email := range emails {
domain, tld = stringsSplit(email)
}
}
_, _ = domain, tld
})
b.Run("strings.SplitN", func(b *testing.B) {
b.ReportAllocs()
b.ResetTimer()
var domain, tld string
for i := 0; i < b.N; i++ {
for _, email := range emails {
domain, tld = stringsSplitN(email)
}
}
_, _ = domain, tld
})
b.Run("strings.Index+strings.Split", func(b *testing.B) {
b.ReportAllocs()
b.ResetTimer()
var domain, tld string
for i := 0; i < b.N; i++ {
for _, email := range emails {
domain, tld = stringsIndexAndStringsSplit(email)
}
}
_, _ = domain, tld
})
b.Run("regexp.Split", func(b *testing.B) {
b.ReportAllocs()
b.ResetTimer()
var domain, tld string
for i := 0; i < b.N; i++ {
for _, email := range emails {
domain, tld = regexpSplit(email)
}
}
_, _ = domain, tld
})
b.Run("strings.Cut", func(b *testing.B) {
b.ReportAllocs()
b.ResetTimer()
var domain, tld string
for i := 0; i < b.N; i++ {
for _, email := range emails {
domain, tld = stringsCut(email)
}
}
_, _ = domain, tld
})

cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
Benchmark
Benchmark/strings.Split
Benchmark/strings.Split-12         	        752	   1563851 ns/op	  640002 B/op	   20000 allocs/op
Benchmark/strings.SplitN
Benchmark/strings.SplitN-12        	        805	   1463329 ns/op	  640003 B/op	   20000 allocs/op
Benchmark/strings.Index+strings.Split
Benchmark/strings.Index+strings.Split-12   1416	    858783 ns/op	  320000 B/op	   10000 allocs/op
Benchmark/regexp.Split
Benchmark/regexp.Split-12                    85	  14605240 ns/op	11088513 B/op	  160024 allocs/op
Benchmark/strings.Cut
Benchmark/strings.Cut-12                   6597	    180579 ns/op	       0 B/op	       0 allocs/op
PASS