When I register a patient, for the Given Name I can input e.g. "_______" or "##$!@3" . The same goes for any text input in the registration form e.g. Address and Relatives. Also for Phone Number I can input "-----------" and will still be marked valid.
For more info, please read the Form Input Validation section on the GDoc https://goo.gl/ZNWSjW .
This ticket might be related: https://issues.openmrs.org/browse/RA-543
We should add more validations to these text fields.