<html>
 <head>
  <title>表单验证</title>
  <script type="text/javascript">
    function valid(){
  var username=document.getElementById("username");
  if(username.value==""){
        alert("请输入用户名");
return;
  }
  if(username.value.length<6){
 alert("用户名至少应该有6个字符");
     return;
  }
      var email=document.getElementById("email");
      var atIndex=email.value.indexOf("@");
      var dotIndex=email.value.indexOf(".");
      if(atIndex==-1 || dotIndex==-1){
    alert("必须要有@和.符号");
return;
  }
      if(atIndex>dotIndex){
  alert("@符号不能在.后面");
  return;
  }
      if(dotIndex-atIndex==1){
          alert("@和.不能相邻");
  return;
  }
      var registerForm=document.getElementById("registerForm");
  //提交表单
      registerForm.submit();
}
  </script>
 </head>
 <body>
 <form id="registerForm" action="http://www.baidu.com" method="post" >
   用户名：<input type="text" name="username" id="username" />
   <br/>
   密码:<input type="password" name="password" id="password" />
   <br/>
   确认密码:<input type="password" name="check_password" id="check_password" />
   <br/>
   邮箱:<input type="text" name="email" id="email" />
   <br/>
   <input type="button"   value="注册" />
   </form>
 </body>
</html>