GUI validation on an IBA attribute value.

1. I have defined an IBA attribute of type string.
   
2. Added this to a "package" object in PDMLink/ProjectLInk 9.1
   
3. When i create a package , i can see this attribute in GUI and can enter a string value.
   
4. My requirement is to validate the string value entered using some validation logic through some java code and if the validation is passed then only submit the package for creation otherwise return back saying the error message.
   

Any idea how can i do this validation on an IBA attribute.

-EK