Swift3 substring几种常用用法

        var str = "12345678"
        let length = str.characters.count
        //截取前四位
        var sub1 = str.substring(to: str.index(str.startIndex, offsetBy: 4))
        
        //截取后2位
        var sub2 = str.substring(from: str.index(str.startIndex, offsetBy: length-2))
        
        //截取中间4位，从第2位开始
        let start = str.index(str.startIndex, offsetBy: 2)
        str = str.substring(from: start)  //取到从第2位开始的所有字符
        let end = str.index(str.startIndex, offsetBy: 4)
        var sub3 = str.substring(to: end)   //从新的字符开始取4位