如果一個列字符串包含在另一個列字符串中，則使用Python一對多合并兩個數據幀

如果來自df1的words列的值包含來自df2的keywords列的值，我有兩個數據幀要合并。我一直在嘗試使用str.extract。但到目前為止，沒有運氣達到預期的結果。示例如下：

df1:

[{'id': 1, 'words': 'chellomedia', 'languages': nan},
 {'id': 2, 'words': 'Moien Welt!', 'languages': 'Luxemburgish'},
 {'id': 3, 'words': 'Ahoj světe!', 'languages': 'Czech'},
 {'id': 4, 'words': 'hello world', 'languages': nan},
 {'id': 5, 'words': '?Hola Mundo!', 'languages': 'Spanish'},
 {'id': 6, 'words': 'hello kitty', 'languages': 'English'},
 {'id': 7, 'words': 'Ciao mondo!', 'languages': 'Italian'},
 {'id': 8, 'words': 'hola world', 'languages': nan}]

df2:

[{'code': 1, 'keywords': 'Hello'},
 {'code': 2, 'keywords': 'hola'},
 {'code': 3, 'keywords': 'world'}]

我的試用代碼：

df1['words'] = df1['words'].str.lower()
df2['keywords'] = df2['keywords'].str.lower()

pat = '|'.join([re.escape(x) for x in df2.keywords])
df1.insert(0, 'keywords', df1['words'].str.extract('(' + pat + ')', expand=False))

pd.merge(df1, df2, on='keywords', how='left')

Out:

  keywords  id         words     languages  code
0    hello   1   chellomedia           NaN   1.0
1      NaN   2   moien welt!  Luxemburgish   NaN
2      NaN   3   ahoj světe!         Czech   NaN
3    hello   4   hello world           NaN   1.0
4     hola   5  ?hola mundo!       Spanish   2.0
5    hello   6   hello kitty       English   1.0
6      NaN   7   ciao mondo!       Italian   NaN
7     hola   8    hola world           NaN   2.0

但是想要的應該是這樣的：

  keywords  id         words     languages  code
0    hello   1   chellomedia           NaN   1.0
1      NaN   2   moien welt!  Luxemburgish   NaN
2      NaN   3   ahoj světe!         Czech   NaN
3    hello   4   hello world           NaN   1.0
4    world   4   hello world           NaN   3.0  ---> should be generated in df
5     hola   5  ?hola mundo!       Spanish   2.0
6    hello   6   hello kitty       English   1.0
7      NaN   7   ciao mondo!       Italian   NaN
8     hola   8    hola world           NaN   2.0
9    world   8    hola world           NaN   3.0  ---> should be generated in df

我如何才能產生預期的結果？謝謝。