<script>
    var chars=[];
    for (var r=48;r<=57;r++)
    {
      chars.push(String.fromCharCode(r));
    }
    for (var min=97;min<=122;min++)
    {
      chars.push(String.fromCharCode(min));
    }
    for (var max=65;max<=90;max++)
    {
      chars.push(String.fromCharCode(max));
    }
    //debugger;
    function getCode(){
      for (var i=0,code=[];i<4;i++)
      {
        var r=Math.floor(Math.random()*62);
        code.push(chars[r]);
      }
      return code.join(" ");
    }
    var code=getCode();
    //debugger;
    var input;
    while((input=prompt("请输入验证码："+code)).toUpperCase()!=code.toUpperCase()){
      alert("验证码输入错误，请重新输入")
      code=getCode();
    }
    document.write("<h3>验证码输入正确</h3>")
  </script>