Series.str.replace does not support $ and ^ regex charecters

[eeshugerman]

Code Sample

import pandas as pd

s = pd.Series(['foo'])

s.str.replace('$', 'bar', regex=True)
# 0    foo
# dtype: object

s.str.replace('^', 'bar', regex=True)
# 0    foo
# dtype: object

Problem description

When regex=True, ^ should match the beginning of the string, and $ should match the end of the string, but this is not the case.

Expected Output

import pandas as pd

s = pd.Series(['foo'])

s.str.replace('$', 'bar', regex=True)
# 0    foobar
# dtype: object

s.str.replace('^', 'bar', regex=True)
# 0    barfoo
# dtype: object

Output of pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.9.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.10-134.current
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : en_US.utf8
LOCALE           : en_US.UTF-8

pandas           : 0.25.3
numpy            : 1.15.4
pytz             : 2018.7
dateutil         : 2.7.5
pip              : 18.1
setuptools       : 40.5.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.7.6.1 (dt dec pq3 ext lo64)
jinja2           : 2.10
IPython          : 7.1.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : 1.2.14
tables           : None
xarray           : None
xlrd             : 1.1.0
xlwt             : None
xlsxwriter       : None

[Liam3851]

I think this is a duplicate of #24804 but it's good to see a case of an attempted meaningful single-character regex in use in the wild (while s + 'bar' is probably more common, in theory this should work).

This is a good reminder that #24804 is still there and this anomaly needs deprecation. Unless, of course, consensus now (given another year has passed and reliance interests on the pre-0.23 behavior are presumably lower) the thought is that this anomaly can just be fixed as opposed to deprecated.

[jreback]

yep this looks like a duplicate thanks @Liam3851

as always PRs are welcome to patch

[eeshugerman]

Ah I see: when the pattern is a single character it's always treated as a literal, regardless of the regex parameter.

Are there plans to make to make regex=False the default? If so, perhaps this could be rolled into the same deprecation warning and API change documentation.

FWIW, my use case: we've built what's basically a lightweight report cleaning app, and the pattern to replace is user input. The app does not have a feature to prepend/append to values; rather the user should be able to accomplish this with the regex find/replace functionality.