注册首页中离焦自动验证

**js代码

 <script> 
  window.function () {
               //给单个表单项绑定onblur事件
               //用户名绑定离焦事件
               var username = document.getElementById("username");
               username.onblur = checkUsername; 
               //   密码绑定离焦事件 	
               	/* var username =   document.getElementById("password");
               username.onblur = checkPassword;
               }*/
   
   function checkUsername(){
           //1.获取用户名的值
           var username = document.getElementById("username").value;
           //2.定义正则表达式
           var reg_username = /^\w{6,12}$/;
           //3.判断值是否符合正则的规则
           var flag = reg_username.test(username);
           //4.提示信息
           var s_username = document.getElementById("s_username");
   
     if(flag){
               //提示绿色对勾
               s_username.innerHTML = "<img   src='自己插入图片'/>";
           }else{
               //提示红色用户名有误
               s_username.innerHTML = "用户名格式有误";
           }
           return flag;
       } </script>

//表中用户名行
*

*<tr>
                        <td >
                            <label for="username"> 用户名</label>
                        </td>
                        <td >
                            <input type="text" name="username" placeholder="请输入用户名" id="username" >
                            <span id="s_username"></span>
                        </td>
    </tr>

**