def _validate_mets_file_group_names(self):
        """
        Ensure ``USE`` attributes of ``mets:fileGrp`` conform to OCR-D naming schema..

        See `spec <https://ocr-d.github.io/mets#file-group-use-syntax>`_.
        """
        for fileGrp in self.mets.file_groups:
            if not fileGrp.startswith(FILE_GROUP_PREFIX):
                self.report.add_notice("fileGrp USE does not begin with '%s': %s" % (FILE_GROUP_PREFIX, fileGrp))
            else:
                # OCR-D-FOO-BAR -> ('FOO', 'BAR')
                # \____/\_/ \_/
                #   |    |   |
                # Prefix |  Name
                #     Category
                category = fileGrp[len(FILE_GROUP_PREFIX):]
                name = None
                if '-' in category:
                    category, name = category.split('-', 1)
                if category not in FILE_GROUP_CATEGORIES:
                    self.report.add_error("Unspecified USE category '%s' in fileGrp '%s'" % (category, fileGrp))
                if name is not None and not re.match(r'^[A-Z0-9-]{3,}$', name):
                    self.report.add_error("Invalid USE name '%s' in fileGrp '%s'" % (name, fileGrp))