<span style="font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">编写自定义的JavaScript函数checkEmail(),在该函数中首先判断E-mail文本框是否为空,然后在应用正则表达式判断E-mail地址是否合法,如果不合法提示用户</span>

1. <span style="font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);"></span><pre name="code" class="javascript"><script type="text/javascript">  
2.         function checkEmail(){  
3.             var email = document.getElementById("email");  
4.             if(email.value == null || email.value == ""){  
5.                 alert("请输入E-mail地址!");  
6.                 email.focus();  
7.                 return;  
8.             }  
9.             var regExp = /w+([-+.]w+)*@w+([-.]w+)*.w+([-.]w+)*/;  
10.             if(!regExp.test(email.value)){  
11.                 alert("您输入的E-mail地址不正确!");  
12.                 email.focus();  
13.                 return;  
14.             }  
15.         }  
16.     </script><span style="white-space:pre"> </span>