input: Write a python function to evaluate a mathematical expression in reverse order.
  assert evaluate_expr("2-(8-3)") == '3'
  assert evaluate_expr("3 + (2 * 2)") == '7'
  assert evaluate_expr("10 / 2 + 3") == '8.0'
output: def evaluate_expr(expr):
      return str(eval(expr[::-1]))