code: def get_extended_attrtext(value):
      
      m = _non_extended_attribute_end_matcher(value)
      if not m:
          raise errors.HeaderParseError(
              "expected extended attrtext but found {!r}".format(value))
      attrtext = m.group()
      value = value[len(attrtext):]
      attrtext = ValueTerminal(attrtext, 'extended-attrtext')
      _validate_xtext(attrtext)
      return attrtext, value
docs: attrtext = 1*(any non-ATTRIBUTE_ENDS character plus '%')
  
      This is a special parsing routine so that we get a value that
      includes % escapes as a single string (which we decode as a single
      string later).