code: def test_regular_span_std():
      primitive_instance = ExponentialWeightedSTD(span=1.5)
      primitive_func = primitive_instance.get_function()
      array = pd.Series([1, 2, 7, 5])
      answer = pd.Series(primitive_func(array))
      correct_answer = pd.Series(
          [
              np.nan,
              0.7071067811865476,
              3.6055512754639887,
              1.7311551816712718,
          ],
      )
      pd.testing.assert_series_equal(answer, correct_answer)
  
imports: import numpy
  import pandas
  from featuretools.primitives import ExponentialWeightedAverage, ExponentialWeightedSTD, ExponentialWeightedVariance
label_desc: Use image node_id 8 for calling a global function with example usage: test_regular_span_std() without return types