DOC: Document that str.match accepts a regular expression

[hamdanal]

Similar to str.fullmatch and other methods that accept regular expressions

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[simonjayhawkins]

by regular expression do you mean a string that is interpreted as a regular expression or a compiled regular expression object?

to avoid confusion, if the former then no doc change probably needed, if the later the type hints in the signature would also need to be updated and some code changes required?

[loicdiridollou]

I think he meant a compiled regular expression, this is how we are trying to type it in the stubs.
I believe we should align all the docs, since it uses the functions of re under the hood the functions below support re.Pattern so compiled regular expression is also accepted at runtime.
If we look at the docs it seems like it is a bit unclear what regular expression means because I would assume it is just a regular string in the for r"...".
So the question is should we allow for compiled regular expression as it is supported at runtime?

• https://pandas.pydata.org/docs/reference/api/pandas.Series.str.rsplit.html
• https://pandas.pydata.org/docs/reference/api/pandas.Series.str.match.html
• https://pandas.pydata.org/docs/reference/api/pandas.Series.str.replace.html
• https://pandas.pydata.org/docs/reference/api/pandas.Series.str.findall.html
• https://pandas.pydata.org/docs/reference/api/pandas.Series.str.extract.html
• https://pandas.pydata.org/docs/reference/api/pandas.Series.str.extractall.html
• https://pandas.pydata.org/docs/reference/api/pandas.Series.str.fullmatch.html

[simonjayhawkins]

the documentation is the official API. If the stubs have been updated to reflect the types that are accepted then this is the tail wagging the dog? If we update the documentation, then we also need to update the type annotations in the code as well as ensure that the behavior is tested?

[loicdiridollou]

I see your point and you are correct, I think the confusion originally came from regular expression != compiled regex.
But then I went into the stubs and it seems like we are testing for it:

pandas/pandas/tests/strings/test_find_replace.py

Lines 562 to 571 in c888af6

	def test_replace_compiled_regex_mixed_object():
	pat = re.compile(r"BAD_*")
	ser = Series(
	["aBAD", np.nan, "bBAD", True, datetime.today(), "fooBAD", None, 1, 2.0]
	)
	result = Series(ser).str.replace(pat, "", regex=True)
	expected = Series(
	["a", np.nan, "b", np.nan, np.nan, "foo", None, np.nan, np.nan], dtype=object
	)
	tm.assert_series_equal(result, expected)

So the question would be to clarify what do we mean by regular expression, is it compiled or not, and so we can:

• clarify the docs
• update the stubs according to allow or not re.Pattern[str]

Please let us know @simonjayhawkins.

[hamdanal]

It is true that I understood regular expression to mean both string and compiled patterns but this PR is meant to bring the match method in line with the other str methods.

I can open another PR to clarify the docs and update the inline types if there is consensus.

[github-actions]

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[simonjayhawkins]

If we update the documentation, then we also need to update the type annotations in the code as well as ensure that the behavior is tested?

Its appears that the change proposed in this PR has been done in #61964 along with updates to type annotations as tests added as suggested.