BUG: inconsistency in dtype of replace()

pandas/core/internals/blocks.py

@@ -731,7 +731,7 @@ def _replace_regex(
         replace_regex(new_values, rx, value, mask)
 
         block = self.make_block(new_values)
-        return [block]
+        return block.convert(numeric=False, copy=False)
 
     @final
     def replace_list(

pandas/tests/frame/methods/test_replace.py

@@ -1503,3 +1503,16 @@ def test_regex_replace_scalar(
 
         expected.loc[expected["a"] == ".", "a"] = expected_replace_val
         tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize("regex", [False, True])
+    def test_replace_regex_dtype_frame(self, regex):
+        # GH-48644
+        df1 = DataFrame({"A": ["0"], "B": ["0"]})
+        expected_df1 = DataFrame({"A": [1], "B": [1]})
+        result_df1 = df1.replace(to_replace="0", value=1, regex=regex)
+        tm.assert_frame_equal(result_df1, expected_df1)
+
+        df2 = DataFrame({"A": ["0"], "B": [np.NaN]})
+        expected_df2 = DataFrame({"A": [1], "B": [np.NaN]})
+        result_df2 = df2.replace(to_replace="0", value=1, regex=regex)
+        tm.assert_frame_equal(result_df2, expected_df2)

pandas/tests/series/methods/test_replace.py

@@ -514,3 +514,11 @@ def test_pandas_replace_na(self):
         result = ser.replace(regex_mapping, regex=True)
         exp = pd.Series(["CC", "CC", "CC-REPL", "DD", "CC", "", pd.NA], dtype="string")
         tm.assert_series_equal(result, exp)
+
+    @pytest.mark.parametrize("regex", [False, True])
+    def test_replace_regex_dtype_series(self, regex):
+        # GH-48644
+        series = pd.Series(["0"])
+        expected = pd.Series([1])
+        result = series.replace(to_replace="0", value=1, regex=regex)
+        tm.assert_series_equal(result, expected)