docs: def validate_id(err, value, source):
      'Tests a manifest UUID value'
  
      field_name = '<em:id>' if source == 'install.rdf' else 'id'
  
      id_pattern = re.compile(
          '('
          # UUID format.
          '\{[0-9a-f]{8}-([0-9a-f]{4}-){3}[0-9a-f]{12}\}'
          '|'
          # "email" format.
          '[a-z0-9-\.\+_]*\@[a-z0-9-\._]+'
          ')',
          re.I)
  
      err.metadata['id'] = value
  
      # Must be a valid UUID string.
      if not id_pattern.match(value):
          err.error(
              ('metadata_helpers', '_test_id', 'invalid'),
              'The value of {name} is invalid'.format(name=field_name),
              ['The values supplied for {name} in the {source} file is not a '
               'valid UUID string or email address.'.format(
                   name=field_name, source=source),
               'For help, please consult: '
               'https://developer.mozilla.org/en/install_manifests#id'],
              source)