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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def update_subject_identifier_on_save(self):
 """Overridden to not set the subject identifier on save. """

        if not self.subject_identifier:
            self.subject_identifier = self.subject_identifier_as_pk.hex
        elif re.match(UUID_PATTERN, self.subject_identifier):
            pass
        return self.subject_identifier