add tests and replace escape

Author: charlesdong1991

doc/source/whatsnew/v0.25.0.rst

@@ -40,7 +40,7 @@ Other API Changes
 Deprecations
 ~~~~~~~~~~~~
 
-- :func:`Series.str.replace`, the default regex for single character might be deprecated.
+- :func:`Series.str.replace`, when pat is single special regex character and regex is not defined, regex is by default False for now, but this might be deprecated in the future.
 
 
 .. _whatsnew_0250.prior_deprecations:

pandas/core/strings.py

@@ -583,7 +583,7 @@ def str_replace(arr, pat, repl, n=-1, case=None, flags=0, regex=None):
                              "regex=False")
         # if regex is default None, and a single special character is given
         # in pat, still take it as a literal, and raise the Future warning
-        if regex is None and len(pat) == 1 and re.escape(pat) != pat:
+        if regex is None and len(pat) == 1 and pat in list("[\^$.|?*+()]"):
             warnings.warn("'{}' is interpreted as a literal in ".format(pat) +
                           "default, not regex. It will change in the future.",
                           FutureWarning)

pandas/tests/test_strings.py

@@ -3441,12 +3441,21 @@ def test_replace_single_pattern(self, regex, expected_array):
         expected = Series(expected_array)
         tm.assert_series_equal(result, expected)
 
-    def test_replace_without_specifying_regex_parameter(self):
-        values = Series(['a.c'])
-        # GH: 24804
-        # test future deprecation warning
-        with tm.assert_produces_warning(FutureWarning,
-                                        check_stacklevel=False):
-            result = values.str.replace('.', 'b')
-        expected = Series(['abc'])
+    @pytest.mark.parametrize("input_array, single_char, replace_char,"
+                             "expect_array, warn", [
+                                ("a.c", ".", "b", "abc", True),
+                                ("a@c", "@", "at", "aatc", False)
+                                ])
+    def test_replace_warning_single_character(self, input_array,
+                                              single_char, replace_char,
+                                              expect_array, warn):
+        values = Series([input_array])
+        if warn:
+            with tm.assert_produces_warning(FutureWarning,
+                                            check_stacklevel=False):
+                result = values.str.replace(single_char, replace_char)
+        else:
+            result = values.str.replace(single_char, replace_char)
+
+        expected = Series([expect_array])
         tm.assert_series_equal(result, expected)