<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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)]