js 验证电子邮箱

//userinfo是from表单名(name),email是文本框名(name)

 if(document.userinfo.email.value.length!=0)  {    if (document.userinfo.email.value.charAt(0)=="." ||                 document.userinfo.email.value.charAt(0)=="@"||                document.userinfo.email.value.indexOf('@', 0) == -1 ||          document.userinfo.email.value.indexOf('.', 0) == -1 ||          document.userinfo.email.value.lastIndexOf("@")==document.userinfo.email.value.length-1 ||          document.userinfo.email.value.lastIndexOf(".")==document.userinfo.email.value.length-1)     {      alert("Email地址格式不正确！");      document.userinfo.email.focus();      return false;      }   }else  {   alert("Email不能为空！");   document.userinfo.email.focus();   return false;   }