input: Write a function to evaluate a given string containing a numerical expression and return the result.
  def eval_expr(expr):
     return eval(expr)
  assert eval_expr("3 + 5 * 2")==13
  assert eval_expr("10 / 2 + 3")==8
  assert eval_expr("4 ** 2 - 1")==15
  assert eval_expr("(7 + 3) * 2")==20
  assert eval_expr("0")==0
output: def eval_expr(expr):
     return eval(expr)