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## Description This PR adds support for custom header patterns in `MarkdownHeaderTextSplitter`, allowing users to define non-standard Markdown header formats (like `**Header**`) and specify their hierarchy levels. **Issue:** Fixes #22738 **Dependencies:** None - this change has no new dependencies **Key Changes:** - Added optional `custom_header_patterns` parameter to support non-standard header formats - Enable splitting on patterns like `**Header**` and `***Header***` - Maintain full backward compatibility with existing usage - Added comprehensive tests for custom and mixed header scenarios ## Example Usage ```python from langchain_text_splitters import MarkdownHeaderTextSplitter headers_to_split_on = [ ("**", "Chapter"), ("***", "Section"), ] custom_header_patterns = { "**": 1, # Level 1 headers "***": 2, # Level 2 headers } splitter = MarkdownHeaderTextSplitter( headers_to_split_on=headers_to_split_on, custom_header_patterns=custom_header_patterns, ) # Now **Chapter 1** is treated as a level 1 header # And ***Section 1.1*** is treated as a level 2 header ``` ## Testing - ✅ Added unit tests for custom header patterns - ✅ Added tests for mixed standard and custom headers - ✅ All existing tests pass (backward compatibility maintained) - ✅ Linting and formatting checks pass --- The implementation provides a flexible solution while maintaining the simplicity of the existing API. Users can continue using the splitter exactly as before, with the new functionality being entirely opt-in through the `custom_header_patterns` parameter. --------- Co-authored-by: Mason Daugherty <mason@langchain.dev> Co-authored-by: Claude <noreply@anthropic.com>