function login() {
     var name = document.form.username.value.length;
    var userpassword = document.form.userpassword.value;
    var patrn=/^(\w){6,20}$/; 
     if(document.form.username.value == ""){
	  alert("请输入用户名！");
	  return false;
	 }
    if(document.form.userpassword.value == ""){
	  alert("请输入密码！");
	  return false;
	 }
     if(name > 80 || name <= 1) {
         alert("用户名只能输入2-80位字符！");
         document.form.username.focus();
       return false;
     }
 if (!patrn.exec(userpassword)) {
	 alert("密码只能输入6-20个字母、数字、下划线！");
	 return false;  
 	}     

     document.form.submit();
}