Golang's LeftStr, RightStr, SubStr

I believe there are no LeftStr(str,n) (take at most n first characters), RightStr(str,n) (take at most n last characters) and SubStr(str,pos,n) (take first n characters after pos) function in Golang, so I tried to make one

// take at most n first characters
func Left(str string, num int) string {
    if num <= 0 {
        return ``
    }
    if num > len(str) {
        num = len(str)
    }
    return str[:num]
}

// take at most last n characters
func Right(str string, num int) string {
    if num <= 0 {
        return ``
    }
    max := len(str)
    if num > max {
        num = max
    }
    num = max - num
    return str[num:]
}

But I believe those functions will give incorrect output when the string contains unicode characters. What's the fastest solution for those function, is using for range loop is the only way?