纯数字的验证码

<html>
    <head>
        <title>纯字验证码</title>
        <meta http-equiv='content-type' content='text/html;charset=utf-8'/>
        <script type='text/javascript'>
        var code ; //在全局定义验证码  
        
        function createCode(){
             code = "";   
             var codeLength = 4;//验证码的长度  
             var checkCode = document.getElementById("code");   
             var random = new Array(0,1,2,3,4,5,6,7,8,9,'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R',  
             'S','T','U','V','W','X','Y','Z');//随机数  
             for(var i = 0; i < codeLength; i++) {//循环操作  
                var index = Math.floor(Math.random()*36);//取得随机数的索引（0~35）  
                code += random[index];//根据索引取得随机数加到code上  
            }  
            checkCode.value = code;//把code值赋给验证码  
        }
        //校验验证码  
        function validate(){  
            var inputCode = document.getElementById("input").value.toUpperCase(); //取得输入的验证码并转化为大写        
            if(inputCode.length <= 0) { //若输入的验证码长度为0  
                alert("请输入验证码！"); //则弹出请输入验证码  
            }else if(inputCode != code ) { //若输入的验证码与产生的验证码不一致时  
                alert("验证码输入错误！@_@"); //则弹出验证码输入错误  
                createCode();//刷新验证码  
                document.getElementById("input").value = "";//清空文本框  
            }else { //输入正确时  
                alert("合格！^-^");
            }
        }
        </script>
        <style type='text/css'>
        #code{
            font-family:Arial,宋体;
            font-style:italic;
            color:green;
            border:0;
            padding:2px 3px;
            letter-spacing:3px;
            font-weight:bolder;
        }
        </style>
    </head>
    <body onload='createCode()'> 
        <div>验证码：  
            <input type = "text" id = "input"/>  
            <input type="button" id="code" onclick="createCode()" style="width:60px" title='点击更换验证码' />
            <input type = "button" value = "验证" onclick = "validate()"/>
        </div>  
    </body>
</html>
效果：
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