From 49eb00afb7e9f0ad6ef5c9bda1a0ff75e5864ae0 Mon Sep 17 00:00:00 2001
From: Kropiunig <48442031+Kropiunig@users.noreply.github.com>
Date: Thu, 27 Aug 2026 01:18:48 +0200
Subject: [PATCH 1/3] fix: make str.replace with n=0 perform no replacements

The regex branch of StringAccessor.replace passed count=max(n, 0) to
re.sub, where count=0 means 'replace every occurrence'. That collapsed
n=0 onto n=-1, so asking for zero replacements silently replaced all of
them, while the regex=False branch (str.replace) honoured n=0 correctly.

Special-case n=0 so both branches match the documented behaviour that n
is the number of replacements to make from the start and only -1 means
'replace all'.
---
 xarray/core/accessor_str.py       |  6 ++++--
 xarray/tests/test_accessor_str.py | 22 ++++++++++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/xarray/core/accessor_str.py b/xarray/core/accessor_str.py
index 0699fbdd5b1..b2f6fd4a897 100644
--- a/xarray/core/accessor_str.py
+++ b/xarray/core/accessor_str.py
@@ -1944,8 +1944,10 @@ def replace(
 
         if regex:
             pat = self._re_compile(pat=pat, flags=flags, case=case)
-            func = lambda x, ipat, irepl, i_n: ipat.sub(
-                repl=irepl, string=x, count=max(i_n, 0)
+            # ``re.sub`` interprets ``count=0`` as "replace every occurrence",
+            # so ``n=0`` has to be special-cased to mean "replace nothing".
+            func = lambda x, ipat, irepl, i_n: (
+                x if i_n == 0 else ipat.sub(repl=irepl, string=x, count=max(i_n, 0))
             )
         else:
             pat = self._stringify(pat)
diff --git a/xarray/tests/test_accessor_str.py b/xarray/tests/test_accessor_str.py
index 0741fa364fc..5f5849bc9de 100644
--- a/xarray/tests/test_accessor_str.py
+++ b/xarray/tests/test_accessor_str.py
@@ -381,6 +381,28 @@ def test_replace(dtype) -> None:
     assert_equal(result, expected)
 
 
+def test_replace_n_zero(dtype) -> None:
+    # ``n=0`` means "make no replacements", for regex and literal patterns alike
+    values = xr.DataArray(["fooBAD__barBAD"], dims=["x"]).astype(dtype)
+
+    result = values.str.replace("BAD[_]*", "", n=0)
+    assert result.dtype == values.dtype
+    assert_equal(result, values)
+
+    result = values.str.replace("BAD", "", n=0, regex=False)
+    assert result.dtype == values.dtype
+    assert_equal(result, values)
+
+    # ``n`` is broadcast, so a single zero must not spill over to its neighbours
+    n = xr.DataArray([0, 1, -1], dims=["y"])
+    result = values.str.replace("BAD[_]*", "", n=n)
+    expected = xr.DataArray(
+        [["fooBAD__barBAD", "foobarBAD", "foobar"]], dims=["x", "y"]
+    ).astype(dtype)
+    assert result.dtype == expected.dtype
+    assert_equal(result, expected)
+
+
 def test_replace_callable() -> None:
     values = xr.DataArray(["fooBAD__barBAD"])
 

From e2ec5fb9c7b20947d0b0b506c976a2558f7682ba Mon Sep 17 00:00:00 2001
From: Kropiunig <48442031+Kropiunig@users.noreply.github.com>
Date: Thu, 27 Aug 2026 01:22:59 +0200
Subject: [PATCH 2/3] docs: add whats-new entry for str.replace n=0 fix

---
 doc/whats-new.rst | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index d1505bfa081..d69deefdd93 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -53,6 +53,11 @@ Deprecations
 Bug Fixes
 ~~~~~~~~~
 
+- Fixed ``DataArray.str.replace`` replacing every occurrence instead of none when
+  ``n=0``. ``re.sub`` treats ``count=0`` as "replace all", so the regex code path
+  collapsed ``n=0`` onto ``n=-1``, while the ``regex=False`` path already handled
+  ``n=0`` correctly.
+  By `Alexander Kropiunig <https://github.com/Kropiunig>`_.
 - Fix async zarr tests using ``wraps`` with ``autospec=True`` on async methods,
   which caused ``AsyncMock`` objects to leak through instead of real array data
   (:pull:`11232`).

From 3c4083334497cdac6f10e7d64c65d93e5ff20b3d Mon Sep 17 00:00:00 2001
From: Kropiunig <48442031+Kropiunig@users.noreply.github.com>
Date: Thu, 27 Aug 2026 01:34:21 +0200
Subject: [PATCH 3/3] docs: reference PR number in whats-new entry

---
 doc/whats-new.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index d69deefdd93..8c807f03dab 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -56,7 +56,7 @@ Bug Fixes
 - Fixed ``DataArray.str.replace`` replacing every occurrence instead of none when
   ``n=0``. ``re.sub`` treats ``count=0`` as "replace all", so the regex code path
   collapsed ``n=0`` onto ``n=-1``, while the ``regex=False`` path already handled
-  ``n=0`` correctly.
+  ``n=0`` correctly (:pull:`11545`).
   By `Alexander Kropiunig <https://github.com/Kropiunig>`_.
 - Fix async zarr tests using ``wraps`` with ``autospec=True`` on async methods,
   which caused ``AsyncMock`` objects to leak through instead of real array data