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")==5.0
  assert eval_expr("5-3+2")==4
  assert eval_expr("3**2")==9
  assert eval_expr("0/1")==0.0
output: def eval_expr(expression):
      return eval(expression)