The only easy way to do something like that would be to limit which groups could create items in specific categories since custom fields are assigned to categories. Other than that, you are looking at some rather complex coding and or some rather wicked template editing. Im also not sure if your focus is on preventing Item Owners from being able to add content to specific fields OR if you are wanting to prevent specific users from viewing specific fields because of a group they are in. That is two completely different sets of circumstances (both would require the same amount of code and or template editing).
You can use both Regular expression and PHP callback for Text fields. That should be all you need to do what you want. You should even be able to use the PHP Callback to prevent specific user groups from adding a URL