How do I write regular expression data from column data to a new column?

>>>df=pd.DataFrame([[abc def], ['123567'], ['qqqeeee']], columns=['a'])
>>df
         a
0abc def
1  123 567
2qqqeeeee
>>>df['a'].str.extract(r'(.{3}$',expand=True)
     0
0def
1  567
2eee
>>>df['b'] = df['a'].str.extract(r'(.{3})$',expand=False)
>>df
         ab
0abc def def def
1  123 567  567
2qqqeeeeeeeee