r, _ := regexp.Compile("m([a-z]+)ch")
fmt.Println(r.FindStringIndex("much match"))
// [0 4]

fmt.Println(r.FindStringSubmatchIndex("much match"))
// [0 4 1 2]

fmt.Println(r.FindAllStringSubmatchIndex("much match", -1))
// [[0 4 1 2] [5 10 6 8]]