Fix inconsistent behavior of `CharacterTextSplitter` when changing `keep_separator` (#7263)

- Description:
- When `keep_separator` is `True` the `_split_text_with_regex()` method
in `text_splitter` uses regex to split, but when `keep_separator` is
`False` it uses `str.split()`. This causes problems when the separator
is a special regex character like `.` or `*`. This PR fixes that by
using `re.split()` in both cases.
- Issue: #7262 
- Tag maintainer: @baskaryan

langchain/text_splitter.py

@@ -47,7 +47,7 @@ def _split_text_with_regex(
                 splits += _splits[-1:]
             splits = [_splits[0]] + splits
         else:
-            splits = text.split(separator)
+            splits = re.split(separator, text)
     else:
         splits = list(text)
     return [s for s in splits if s != ""]

tests/unit_tests/test_text_splitter.py

@@ -80,6 +80,31 @@ def test_character_text_splitter_longer_words() -> None:
     assert output == expected_output
 
 
+def test_character_text_splitter_keep_separator_regex() -> None:
+    """Test splitting by characters while keeping the separator
+    that is a regex special character.
+    """
+    text = "foo.bar.baz.123"
+    splitter = CharacterTextSplitter(
+        separator=r"\.", chunk_size=1, chunk_overlap=0, keep_separator=True
+    )
+    output = splitter.split_text(text)
+    expected_output = ["foo", ".bar", ".baz", ".123"]
+    assert output == expected_output
+
+
+def test_character_text_splitter_discard_separator_regex() -> None:
+    """Test splitting by characters discarding the separator
+    that is a regex special character."""
+    text = "foo.bar.baz.123"
+    splitter = CharacterTextSplitter(
+        separator=r"\.", chunk_size=1, chunk_overlap=0, keep_separator=False
+    )
+    output = splitter.split_text(text)
+    expected_output = ["foo", "bar", "baz", "123"]
+    assert output == expected_output
+
+
 def test_character_text_splitting_args() -> None:
     """Test invalid arguments."""
     with pytest.raises(ValueError):