docs: def extract_version_tag_from_url(url):
      """Extract a DataONE API version tag from a MN or CN service endpoint URL.
  
      Args:
        url : str
          Service endpoint URL. E.g.: ``https://mn.example.org/path/v2/object/pid``.
  
      Returns:
        str : Valid version tags are currently ``v1`` or ``v2``.
  
      """
      m = re.match(r'(/|^)(v\d)(/|$)', url)
      if not m:
          return None
      return m.group(2)