Hi Kuba,
Thanks a lot for the info and the sample code. It was great help. I used your sample code for validating username filed in my registration form. I just added the following line
$this->view->title = "Submit Success"; just before the
$request = $this->getRequest();....rest of the codes are same as the sample code.

But, the validation did not occur. Whether I enter or do not enter any data in the username field, it does not give any error and takes me to the Add Success page. I think I am missing something here...Could you please help me with this?

~Nwim