BUG (string): str.replace with negative n (pandas-dev#59628)

* BUG (string): str.replace with negative n

* update GH ref

Author: jbrockmendel

doc/source/whatsnew/v2.3.0.rst

@@ -102,7 +102,7 @@ Conversion
 
 Strings
 ^^^^^^^
--
+- Bug in :meth:`Series.str.replace` when ``n < 0`` for :class:`StringDtype` with ``storage="pyarrow"`` (:issue:`59628`)
 -
 
 Interval

pandas/core/arrays/string_arrow.py

@@ -352,9 +352,7 @@ def _str_replace(
             fallback_performancewarning()
             return super()._str_replace(pat, repl, n, case, flags, regex)
 
-        func = pc.replace_substring_regex if regex else pc.replace_substring
-        result = func(self._pa_array, pattern=pat, replacement=repl, max_replacements=n)
-        return type(self)(result)
+        return ArrowExtensionArray._str_replace(self, pat, repl, n, case, flags, regex)
 
     def _str_repeat(self, repeats: int | Sequence[int]):
         if not isinstance(repeats, int):

pandas/tests/extension/test_arrow.py

@@ -1840,6 +1840,17 @@ def test_str_replace_negative_n():
     expected = pd.Series(["bc", ""], dtype=ArrowDtype(pa.string()))
     tm.assert_series_equal(expected, actual)
 
+    # Same bug for pyarrow-backed StringArray GH#59628
+    ser2 = ser.astype(pd.StringDtype(storage="pyarrow"))
+    actual2 = ser2.str.replace("a", "", -3, True)
+    expected2 = expected.astype(ser2.dtype)
+    tm.assert_series_equal(expected2, actual2)
+
+    ser3 = ser.astype(pd.StringDtype(storage="pyarrow", na_value=np.nan))
+    actual3 = ser3.str.replace("a", "", -3, True)
+    expected3 = expected.astype(ser3.dtype)
+    tm.assert_series_equal(expected3, actual3)
+
 
 def test_str_repeat_unsupported():
     ser = pd.Series(["abc", None], dtype=ArrowDtype(pa.string()))