BUG: Replace on Series/DataFrame stops replacing after first NA

[asishm]

• closes BUG: DataFrame.replace with regex on StringDtype column with NA values stops replacing after first NA #56599
• 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.

The issue was the line a = a[mask] was triggered only for ndarray and doesn't hit when dtype='string', but the np.place logic applied as long as the result was an ndarray.

The old behavior had things like (depending on the number and location of NAs)

In [2]: s = pd.Series(['m', 'm', pd.NA, 'm', 'm', 'm'], dtype='string')

In [3]: s.replace({'m': 't'}, regex=True)
Out[3]:
0       t
1       t
2    <NA>
3       m
4       t
5       t
dtype: string

In [4]: s = pd.Series(['m', 'm', pd.NA, pd.NA, 'm', 'm', 'm'], dtype='string')

In [5]: s.replace({'m': 't'}, regex=True)
Out[5]:
0       t
1       t
2    <NA>
3    <NA>
4       m
5       m
6       t
dtype: string

In [6]: s = pd.Series(['m', 'm', pd.NA, 'm', 'm', pd.NA, 'm', 'm'], dtype='string')

In [7]: s.replace({'m': 't'}, regex=True)
Out[7]:
0       t
1       t
2    <NA>
3       m
4       t
5    <NA>
6       t
7       m
dtype: string

[asishm]

pre-commit.ci autofix

[mroeschke]

Thanks @asishm

[rhshadrach]

Along with other string type fixes, I think it'd be good to backport this to 2.3.x. Any objection @mroeschke @jorisvandenbossche?

[mroeschke]

No objection from me

[rhshadrach]

@meeseeksdev backport to 2.3.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 2.3.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 0f7ded2a3a637b312f6ad454fd6c0b89d3d3e7aa

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #57865: BUG: Replace on Series/DataFrame stops replacing after first NA'

4. Push to a named branch:

git push YOURFORK 2.3.x:auto-backport-of-pr-57865-on-2.3.x

5. Create a PR against branch 2.3.x, I would have named this PR:

"Backport PR #57865 on branch 2.3.x (BUG: Replace on Series/DataFrame stops replacing after first NA)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.