package main

import (
    "fmt"
    "regexp"
)

func main() {
    re := regexp.MustCompile(`[mrak]+`)
    fmt.Println(re.MatchString("mark")) //true
    fmt.Println(re.MatchString("dave")) //ALSO TRUE?????????
    fmt.Println(re.MatchString("gophergophergopher")) //HOW IS THIS TRUE
}

^[mrak]+$

package main

import (
    "regexp"
    "fmt"
)

func main() {
    var re = regexp.MustCompile(`(?m)^[mrak]+$`)
    var str = `mark
kmrrkamkka
dave
gophergophergopher`

    for i, match := range re.FindAllString(str, -1) {
        fmt.Println(match, "found at index", i)
    }
}