<script type="text/javascript">
    function check()
    {
 
        var theForm = document.forms['aspnetForm'];
  
 
        if(theForm.ctl00$ContentPlaceHolder1$txtUserName.value=="")
        {
            alert("请输入用户名！");
            theForm.ctl00$ContentPlaceHolder1$txtUserName.focus();
            return false
        }           
       
        var myRegex = /@.*\.[a-z]{2,6}/;
        var email = theForm.ctl00$ContentPlaceHolder1$txtUserName.value;
        email = email.replace(/^ | $/g,"");
        email = email.replace(/^\.*|\.*$/g,"");
        email = email.toLowerCase();       
      
        if (!myRegex.test(email))
        {
            alert ("请输入有效的E-Mail！");
            theForm.ctl00$ContentPlaceHolder1$txtUserName.focus();
            return false;
        }
   
        if(theForm.ctl00$ContentPlaceHolder1$txtPassword.value=="")
        {
            alert("请输入密码！");
            theForm.ctl00$ContentPlaceHolder1$txtPassword.focus();
            return false;
        }
       
        return true;
    
       
    }
   
    function checksearch()
    {
        var Form = document.forms['aspnetForm'];
        if(Form.ctl00$ContentPlaceHolder1$SearchText.focus)
        {
            if(Form.ctl00$ContentPlaceHolder1$SearchText.value=="")
            {      
                alert("请输入关键字！");
                Form.ctl00$ContentPlaceHolder1$SearchText.focus();
                return false;
            }
            return true;  
        }
    }
    </script>

后台：

          //添加属性，JS代码用来判断回车键
            SearchText.Attributes.Add("onkeydown", "if (event.keyCode == 13){aspnetForm.ctl00$ContentPlaceHolder1$btnSearch.click();return false;;return false;}");

该文章转载自德仔工作室：http://www.dezai.cn/Article_Show.asp?ArticleID=26245