r.extract(r'(?P<letter>[ab])(?P<digit>\d)')
        letter digit
        0      a     1
        1      b     2
        2    NaN   NaN

        A pattern with one group will return a DataFrame with one column
        if expand=True.

        >>> s.str.extract(r'[ab](\d)', expand=True)
            0
        0    1
        1    2
        2  NaN

        A pattern with one group will return a Series if expand=False.

        >>> s.str.extract(r'[ab](\d)', expand=False)
        0      1
        1      2
        2    NaN
        dtype: object
        r