Add validation check on fields in user form

[t_zaharia]

Hello there

We want to add some extra fields in the user details page. In particular, I added a radio button and according to that selection I show/hide (using jQuery and css) the fields I want to show/hide.
The thing is, we want to add a validation check on one of the fields, meaning if a user inserts a non-acceptable value, he will be shown a corresponding message.
I added the php code and script in edit-address.php and added an id in the table and tr in the edit_address_userfields.php.
My question is: ok, I made them appear, where should I place the code for validation?
As you can see, I am not an experienced developer...I am working on it....

I would appreciate any help on this...

Thank you in advance,
Theoni