code: def get_substring_idxs(substr, string):
      
      return [match.start() for match in re.finditer(substr, string)]
docs: Return a list of indexes of substr. If substr not found, list is
      empty.
  
      Arguments:
          substr (str): Substring to match.
          string (str): String to match in.
  
      Returns:
          list of int: Start indices of substr.