>>> # the following three have the same result >>> pd.Series(['a', 'b', 'aa'], dtype='category').str.extractall(r'(a)') >>> pd.Series(['a', 'b', 'aa']).str.extractall(r'(a)') >>> pd.Index(['a', 'b', 'aa']).str.extractall(r'(a)') 0 match 0 0 a 2 0 a 1 a >>> pd.Index(['a', 'b', 'aa'], dtype='category').str.extractall(r'(a)') Traceback (most recent call last): File "<stdin>", line 1, in <module> File "C:\ProgramData\Miniconda3\envs\pandas-dev\lib\site-packages\pandas\core\strings.py", line 2567, in extractall return str_extractall(self._orig, pat, flags=flags) File "C:\ProgramData\Miniconda3\envs\pandas-dev\lib\site-packages\pandas\core\strings.py", line 1012, in str_extractall is_mi = arr.index.nlevels > 1 AttributeError: 'CategoricalIndex' object has no attribute 'index'