import re


def get_substring_idxs(substr, string):
  """
  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.
  """
  return [match.start() for match in re.finditer(substr, string)]