My website is adding some more field to the registration form and I need to write some code to check it. But can someone help me about the program structure in CB?

I put some code in registerForm() in comprofiler.html.php so it echo some error if users enter wrong value in the form but the program still proceed to Succesful Registration page no matter is the value true or false.
Where should I put some code to tell the browser to go to Registration form again if the value they enter in my fields are not correct?
Thanks in advance