BUG: to_offset doesn't use the compiled regex

[Mulugruntz]

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

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

• [ x] (optional) I have confirmed this bug exists on the master branch of pandas.

---

Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Problem description

When checking lines: https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/tslibs/offsets.pyx#L3582
and https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/tslibs/offsets.pyx#L3456

we can see that there is:

opattern = re.compile(
    r"([+\-]?\d*|[+\-]?\d*\.\d*)\s*([A-Za-z]+([\-][\dA-Za-z\-]+)?)"
)

and

split = re.split(opattern, freq)

Which shows there's a compiled regex but it's not used.

Expected Output

Instead, it should be:

split = opattern.split(freq)

If you ever like this kind of thing...

>>> timeit.timeit("opattern.split(freq)", setup='''import re;opattern = re.compile(r"([+\-]?\d*|[+\-]?\d*\.\d*)\s*([A-Za-z]+([\-][\dA-Za-z\-]+)?)");freq="1Min"''', number=1000000)
0.6493891229999917
>>> timeit.timeit("re.split(opattern, freq)", setup='''import re;opattern = re.compile(r"([+\-]?\d*|[+\-]?\d*\.\d*)\s*([A-Za-z]+([\-][\dA-Za-z\-]+)?)");freq="1Min"''', number=1000000)
1.9217748190000066

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 9d598a5
python : 3.9.1.final.0
python-bits : 64
OS : Darwin
OS-release : 18.6.0
Version : Darwin Kernel Version 18.6.0: Thu Apr 25 23:16:27 PDT 2019; root:xnu-4903.261.4~2/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.UTF-8

pandas : 1.2.1
numpy : 1.20.1
pytz : 2021.1
dateutil : 2.8.1
pip : 20.3.3
setuptools : 51.1.1
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.3
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None