Precise Sentence Boundary Detection for Legal Text

The ALEA Institute has released two specialized libraries for legal sentence boundary detection: NUPunkt and CharBoundary, along with a comprehensive benchmark dataset.

Try Our Interactive Visualization: Explore our interactive demonstration site to visualize how different models handle complex legal sentences in real-time.

These tools solve a critical problem in legal natural language processing: accurately identifying where sentences begin and end in complex legal documents containing specialized citations, abbreviations, and intricate sentence structures.

Why Sentence Boundary Detection Matters

Accurate sentence boundary detection is the foundation of many downstream NLP tasks, particularly for retrieval-augmented generation (RAG) systems in the legal domain. When legal text is incorrectly segmented:

• Critical concepts spanning sentence boundaries get fragmented
• Citations and references lose their context
• Retrieval performance degrades significantly
• RAG systems produce less accurate outputs

Our research shows that each percentage improvement in precision yields exponentially greater reductions in context fragmentation for legal document analysis.

Two Complementary Approaches

NUPunkt

• Pure Python library with zero external dependencies
• Achieves 91.1% precision
• Millions of characters per second on consumer CPU hardware
• 29-32% precision improvement over standard tools
• MIT licensed and easy to integrate

CharBoundary

• Character-level machine learning models
• Three model sizes (small, medium, large)
• Large model achieves highest F1 score of 0.782
• Hundreds of thousands of tokens per second on consumer CPU hardware
• ONNX acceleration for production environments

Benchmark Dataset

To enable further research, we’ve released a comprehensive benchmark dataset for legal sentence and paragraph boundary detection:

• 45,739 text examples
• 107,346 sentence tags and 97,667 paragraph tags
• Derived from the KL3M legal document corpus
• CC BY licensing for the data and models
• MIT licensing for the source code

Related Resources

• Interactive Visualization - Compare models in real-time
• Research Paper on arXiv
• NUPunkt GitHub Repository
• CharBoundary GitHub Repository
• Benchmark Dataset on HuggingFace
• Blog Post: Improving Legal Text Analysis