BUG: Using `Series.str.fullmatch()` and `Series.str.match()` with a compiled regex fails with arrow strings

[Dr-Irv]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd
import re
DATA = ["applep", "bananap", "Cherryp", "DATEp", "eGGpLANTp", "123p", "23.45p"]
s=pd.Series(DATA)
s.str.fullmatch(re.compile(r"applep"))
s.str.match(re.compile(r"applep"))
sa=pd.Series(DATA, dtype="string[pyarrow]")
sa.str.fullmatch(re.compile(r"applep"))
sa.str.match(re.compile(r"applep"))

Issue Description

with pyarrow strings, the last line fails with:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Condadirs\envs\pandasstubs311\Lib\site-packages\pandas\core\strings\accessor.py", line 140, in wrapper
    return func(self, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Condadirs\envs\pandasstubs311\Lib\site-packages\pandas\core\strings\accessor.py", line 1429, in fullmatch
    result = self._data.array._str_fullmatch(pat, case=case, flags=flags, na=na)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Condadirs\envs\pandasstubs311\Lib\site-packages\pandas\core\arrays\_arrow_string_mixins.py", line 320, in _str_fullmatch
    if not pat.endswith("$") or pat.endswith("\\$"):
           ^^^^^^^^^^^^
AttributeError: 're.Pattern' object has no attribute 'endswith'
>>> sa.str.match(re.compile(r"applep"))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Condadirs\envs\pandasstubs311\Lib\site-packages\pandas\core\strings\accessor.py", line 140, in wrapper
    return func(self, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Condadirs\envs\pandasstubs311\Lib\site-packages\pandas\core\strings\accessor.py", line 1388, in match
    result = self._data.array._str_match(pat, case=case, flags=flags, na=na)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Condadirs\envs\pandasstubs311\Lib\site-packages\pandas\core\arrays\_arrow_string_mixins.py", line 309, in _str_match
    if not pat.startswith("^"):
           ^^^^^^^^^^^^^^
AttributeError: 're.Pattern' object has no attribute 'startswith'

Expected Behavior

No exception

Installed Versions

INSTALLED VERSIONS

commit : c888af6
python : 3.11.9
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.26100
machine : AMD64
processor : Intel64 Family 6 Model 183 Stepping 1, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : English_United States.1252

pandas : 2.3.1
numpy : 2.3.1
pytz : 2024.1
dateutil : 2.9.0.post0
pip : 24.2
Cython : None
sphinx : None
IPython : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.13.4
blosc : None
bottleneck : 1.4.2
dataframe-api-compat : None
fastparquet : None
fsspec : None
html5lib : 1.1
hypothesis : None
gcsfs : None
jinja2 : 3.1.6
lxml.etree : 6.0.0
matplotlib : 3.10.3
numba : None
numexpr : 2.10.1
odfpy : None
openpyxl : 3.1.5
pandas_gbq : None
psycopg2 : None
pymysql : None
pyarrow : 20.0.0
pyreadstat : 1.3.0
pytest : 8.4.1
python-calamine : None
pyxlsb : 1.0.10
s3fs : None
scipy : 1.16.0
sqlalchemy : 2.0.41
tables : 3.10.1
tabulate : 0.9.0
xarray : 2025.6.1
xlrd : 2.0.2
xlsxwriter : 3.2.5
zstandard : None
tzdata : 2025.2
qtpy : None
pyqt5 : None

[khemkaran10]

take

[jorisvandenbossche]

FWIW, we don't actually document (or test, I think) that this is supported. But because pat is passed to re.compile(..) in

pandas/pandas/core/strings/object_array.py

Lines 249 to 259 in e4a03b6

	def _str_match(
	self,
	pat: str,
	case: bool = True,
	flags: int = 0,
	na: Scalar | lib.NoDefault = lib.no_default,
	):
	if not case:
	flags |= re.IGNORECASE
	regex = re.compile(pat, flags=flags)

this works (since re.compile accepts that).

Given this works currently, I think it is certainly a good idea to keep this working for the string dtype as well. But then probably also should update the typing and docs.