Add regex control over separators in character text splitter (#7933)

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#7854

Added the ability to use the `separator` ase a regex or a simple
character.
Fixed a bug where `start_index` was incorrectly counting from -1.

Who can review?
@eyurtsev
@hwchase17 
@mmz-001

docs/snippets/modules/data_connection/document_transformers/text_splitters/character_text_splitter.mdx

@@ -12,6 +12,7 @@ text_splitter = CharacterTextSplitter(
     chunk_size = 1000,
     chunk_overlap  = 200,
     length_function = len,
+    is_separator_regex = False,
 )
 ```
 

docs/snippets/modules/data_connection/document_transformers/text_splitters/recursive_text_splitter.mdx

@@ -16,6 +16,7 @@ text_splitter = RecursiveCharacterTextSplitter(
     chunk_size = 100,
     chunk_overlap  = 20,
     length_function = len,
+    is_separator_regex = False,
 )
 ```
 

libs/langchain/langchain/text_splitter.py

@@ -281,15 +281,21 @@ class TextSplitter(BaseDocumentTransformer, ABC):
 class CharacterTextSplitter(TextSplitter):
     """Splitting text that looks at characters."""
 
-    def __init__(self, separator: str = "\n\n", **kwargs: Any) -> None:
+    def __init__(
+        self, separator: str = "\n\n", is_separator_regex: bool = False, **kwargs: Any
+    ) -> None:
         """Create a new TextSplitter."""
         super().__init__(**kwargs)
         self._separator = separator
+        self._is_separator_regex = is_separator_regex
 
     def split_text(self, text: str) -> List[str]:
         """Split incoming text and return chunks."""
         # First we naively split the large input into a bunch of smaller ones.
-        splits = _split_text_with_regex(text, self._separator, self._keep_separator)
+        separator = (
+            self._separator if self._is_separator_regex else re.escape(self._separator)
+        )
+        splits = _split_text_with_regex(text, separator, self._keep_separator)
         _separator = "" if self._keep_separator else self._separator
         return self._merge_splits(splits, _separator)
 
@@ -629,11 +635,13 @@ class RecursiveCharacterTextSplitter(TextSplitter):
         self,
         separators: Optional[List[str]] = None,
         keep_separator: bool = True,
+        is_separator_regex: bool = False,
         **kwargs: Any,
     ) -> None:
         """Create a new TextSplitter."""
         super().__init__(keep_separator=keep_separator, **kwargs)
         self._separators = separators or ["\n\n", "\n", " ", ""]
+        self._is_separator_regex = is_separator_regex
 
     def _split_text(self, text: str, separators: List[str]) -> List[str]:
         """Split incoming text and return chunks."""
@@ -642,15 +650,18 @@ class RecursiveCharacterTextSplitter(TextSplitter):
         separator = separators[-1]
         new_separators = []
         for i, _s in enumerate(separators):
+            _separator = _s if self._is_separator_regex else re.escape(_s)
             if _s == "":
                 separator = _s
                 break
-            if re.search(_s, text):
+            if re.search(_separator, text):
                 separator = _s
                 new_separators = separators[i + 1 :]
                 break
 
-        splits = _split_text_with_regex(text, separator, self._keep_separator)
+        _separator = separator if self._is_separator_regex else re.escape(separator)
+        splits = _split_text_with_regex(text, _separator, self._keep_separator)
+
         # Now go merging things, recursively splitting longer texts.
         _good_splits = []
         _separator = "" if self._keep_separator else separator
@@ -680,7 +691,7 @@ class RecursiveCharacterTextSplitter(TextSplitter):
         cls, language: Language, **kwargs: Any
     ) -> RecursiveCharacterTextSplitter:
         separators = cls.get_separators_for_language(language)
-        return cls(separators=separators, **kwargs)
+        return cls(separators=separators, is_separator_regex=True, **kwargs)
 
     @staticmethod
     def get_separators_for_language(language: Language) -> List[str]:

libs/langchain/tests/unit_tests/test_text_splitter.py

@@ -1,4 +1,5 @@
 """Test text splitting functionality."""
+import re
 from typing import List
 
 import pytest
@@ -80,25 +81,43 @@ def test_character_text_splitter_longer_words() -> None:
     assert output == expected_output
 
 
-def test_character_text_splitter_keep_separator_regex() -> None:
+@pytest.mark.parametrize(
+    "separator, is_separator_regex", [(re.escape("."), True), (".", False)]
+)
+def test_character_text_splitter_keep_separator_regex(
+    separator: str, is_separator_regex: bool
+) -> 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
+        separator=separator,
+        chunk_size=1,
+        chunk_overlap=0,
+        keep_separator=True,
+        is_separator_regex=is_separator_regex,
     )
     output = splitter.split_text(text)
     expected_output = ["foo", ".bar", ".baz", ".123"]
     assert output == expected_output
 
 
-def test_character_text_splitter_discard_separator_regex() -> None:
+@pytest.mark.parametrize(
+    "separator, is_separator_regex", [(re.escape("."), True), (".", False)]
+)
+def test_character_text_splitter_discard_separator_regex(
+    separator: str, is_separator_regex: bool
+) -> 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
+        separator=separator,
+        chunk_size=1,
+        chunk_overlap=0,
+        keep_separator=False,
+        is_separator_regex=is_separator_regex,
     )
     output = splitter.split_text(text)
     expected_output = ["foo", "bar", "baz", "123"]