input: Write a function to evaluate a given numerical expression in string format and return the result.
  
  assert eval_expr("2+3*4") == 14
  assert eval_expr("10/2-3") == 2.0
  assert eval_expr("5**2+1") == 26
  assert eval_expr("8/4*2") == 4.0
  assert eval_expr("7+3-1*4") == 6
output: def eval_expr(expr):
      return eval(expr)