input: Write a function to evaluate a numerical expression given as a string and return the result. 
  assert eval_expr("2+3*4") == 14
  assert eval_expr("5-2") == 3
  assert eval_expr("10/2") == 5
  assert eval_expr("2**3") == 8
  assert eval_expr("0+0") == 0
output: def eval_expr(expression):
      return eval(expression)