input: Write a function to evaluate a numerical expression represented as a string and return the result of the expression.
  assert eval_expr("3 + 4 * 2")==11
  assert eval_expr("(10 + 2) / 4")==3.0
  assert eval_expr("5 ** 2 - 3 * 4")==13
  assert eval_expr("2 * (3 + 4)")==14
  assert eval_expr("7 - 5 / 2")==4.5
output: def eval_expr(expr):
      return eval(expr)