input: Write a function to evaluate a given string containing a numerical expression and return the result.
  assert eval_expr("2+3*4")==14
  assert eval_expr("10/2")==5
  assert eval_expr("7-3")==4
  assert eval_expr("12*3")==36
  assert eval_expr("5**2")==25
output: def eval_expr(expression):
      return eval(expression)