code: def parse_format(self):
          
          if (self.format is not None and
                  not re.match(r'''\w{1,20}$''', self.format)):
              raise IIIFRequestError(
                  parameter='format',
                  text='Bad format parameter')
docs: Check format parameter.
  
          All formats values listed in the specification are lowercase
          alphanumeric value commonly used as file extensions. To leave
          opportunity for extension here just do a limited sanity check
          on characters and length.