3
I would first try to edit the language file and see if that will help you. Look in languages/english/user.php
This looks like the language file that sets the wording for the registration page. Edit the text from just "username" to " enter your username in the this specific format".
I would try that first. If that doesn't work for you, I would then edit the register.php file to add your content. I am not sure how proficient you are with php, but I would not start plugging stuff into that file without some confidence you know what you are doing.
-Matt