def get_innermost_rightmost_group(expression, group_start_char='(',
    group_end_char=')'):
    """
    Given an expression and group start/end identifier characters,
    returns the innermost rightmost group. For example:
    '(2-1)+((3-4)-(2*5))', '(', ')' results in '2*5'
    """
    start_index = expression.rfind(group_start_char)
    end_index = start_index + expression[start_index:].find(group_end_char)
    return expression[start_index + 1:end_index]
