function check_login(){
 if(document.login_form.email.value.length<1){
   alert("E-mail can not be empty!");
   document.login_form.email.focus();
   return (false);  
}
 var filter=/^\s*([A-Za-z0-9_-]+(\.\w+)*@(\w+\.)+\w{2,3})\s*$/;
 if(!filter.test(document.login_form.email.value)){
   alert("E-mail format error!");
   document.login_form.email.focus();
   return (false); 
}
 if(document.login_form.password.value.length<6||document.login_form.password.value.length>12){
   alert("Password median error(6-12 bit)!");
   document.login_form.password.focus();
   return (false);  
}
}