BUG: inconsistency in dtype of replace()

pandas/core/internals/blocks.py

@@ -663,7 +663,9 @@ def replace(
         regex = should_use_regex(regex, to_replace)
 
         if regex:
-            return self._replace_regex(to_replace, value, inplace=inplace)
+            self.values = np.asarray(
+                self._replace_regex(to_replace, value, inplace=inplace)
+            )
 
         if not self._can_hold_element(to_replace):
             # We cannot hold `to_replace`, so we know immediately that

pandas/tests/series/methods/test_replace.py

@@ -512,3 +512,10 @@ 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)
+
+    def test_replace_regex_dtype(self):
+        # GH-48644
+        s = pd.Series(["0"])
+        exp = s.replace(to_replace="0", value=1, regex=False).dtype
+        res = s.replace(to_replace="0", value=1, regex=True).dtype
+        assert exp == res