Pandas Series: str.extractall() function

Series-str.extractall() function

The str.extractall() function is used to extract groups from all matches of regular expression pat.

For each subject string in the Series, extract groups from all matches of regular expression pat. When each subject string in the Series has exactly one match, extractall(pat).xs(0, level=’match’) is the same as extract(pat).

Syntax:

Series.str.extractall(self, pat, flags=0)

Parameters:

Returns: A DataFrame with one row for each match, and one column for each group. Its rows have a MultiIndex with first levels that come from the subject Series. The last level is named 'match' and indexes the matches in each item of the Series. Any capture group names in regular expression pat will be used for column names, otherwise capture group numbers will be used.

Example - A pattern with one group will return a DataFrame with one column. Indices with no matches will not appear in the result:

Python-Pandas Code:

import numpy as np
import pandas as pd
s = pd.Series(["a2a3", "b2", "c2"], index=["X", "Y", "Z"])
s.str.extractall(r"[ab](\d)")

Output:

        0
  match	
X	 0	  2
1	 3
Y	 0	  2

Example - Capture group names are used for column names of the result:

Python-Pandas Code:

import numpy as np
import pandas as pd
s = pd.Series(["a2a3", "b2", "c2"], index=["X", "Y", "Z"])
s.str.extractall(r"[ac](?P<digit>\d)")

Output:

        digit
  match	
X	  0	     2
    1	     3
Z	  0	     2

Example - A pattern with two groups will return a DataFrame with two columns:

Python-Pandas Code:

import numpy as np
import pandas as pd
s = pd.Series(["a2a3", "b2", "c2"], index=["X", "Y", "Z"])
s.str.extractall(r"(?P<letter>[ac])(?P<digit>\d)")

Output:

         letter	digit
  match		
X	 0	    a	    2
1	        a	    3
Z	 0	    c	    2

Example - Optional groups that do not match are NaN in the result:

Python-Pandas Code:

import numpy as np
import pandas as pd
s = pd.Series(["a2a3", "b2", "c2"], index=["X", "Y", "Z"])
s.str.extractall(r"(?P<letter>[ac])?(?P<digit>\d)")

Output:

          letter	digit
  match		
X	 0	       a	2
   1         a	3
Y	 0	       NaN	2
Z	 0	       c	2

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