Parse & Extract : unable to use a custom regex

[simonaubertbd]

Hello,

I tried to use a custom regex by adding it :

Result :

Error

pattern contains no capture groups
Show Traceback

---

ValueError Traceback (most recent call last)
Cell In[9], line 20
16 inlineInput1 = pd.read_csv(StringIO(inlineInput1_data)).convert_dtypes()
19 # Extract data using regex
---> 20 extract2_extracted = inlineInput1['LastName'].str.extract(r"^[a-zA-Z]+$")
21 extract2_extracted.columns = []
22 extract2 = inlineInput1.join(extract2_extracted, rsuffix="_extracted")

File ~\AppData\Local\Programs\Python\Python312\Lib\site-packages\pandas\core\strings\accessor.py:137, in forbid_nonstring_types.._forbid_nonstring_types..wrapper(self, *args, **kwargs)
132 msg = (
133 f"Cannot use .str.{func_name} with values of "
134 f"inferred dtype '{self._inferred_dtype}'."
135 )
136 raise TypeError(msg)
--> 137 return func(self, *args, **kwargs)

File ~\AppData\Local\Programs\Python\Python312\Lib\site-packages\pandas\core\strings\accessor.py:2740, in StringMethods.extract(self, pat, flags, expand)
2738 regex = re.compile(pat, flags=flags)
2739 if regex.groups == 0:
-> 2740 raise ValueError("pattern contains no capture groups")
2742 if not expand and regex.groups > 1 and isinstance(self._data, ABCIndex):
2743 raise ValueError("only one regex group is supported with Index")

ValueError: pattern contains no capture groups

Not sure I use it right (maybe a documentation issue?)

Best regards,

Simon

[tgourdel]

Hi Simon, yes indeed in Python, regex expects the "caputre group" which is the parentheses to let the program know where to catch the value or values, in your case:
^([a-zA-Z]+)$

definitely a documentation issue!

[simonaubertbd]

@tgourdel And I'm definitely not a Python dev ! ;)