code: def find_version_by_regex(
      file_source, version_token="__version__"
  ):  # type: (str,str)->Optional[str]
      
      if not file_source:
          return None
      version_match = re.search(
          r"^" + version_token + r" = ['\"]([^'\"]*)['\"]", file_source, re.M
      )
      if version_match:
          candidate = version_match.group(1)
          if candidate == "" or candidate == ".":  # yes, it will match to a .
              return None
          return candidate
      return None
docs: Regex for dunder version