7.7   Summary 小结

• Information extraction systems search large bodies of unrestricted text for specific types of entities and relations, and use them to populate well-organized databases. These databases can then be used to find answers for specific questions.
• The typical architecture for an information extraction system begins by segmenting, tokenizing, and part-of-speech tagging the text. The resulting data is then searched for specific types of entity. Finally, the information extraction system looks at entities that are mentioned near one another in the text, and tries to determine whether specific relationships hold between those entities.
• Entity recognition is often performed using chunkers, which segment multi-token sequences, and label them with the appropriate entity type. Common entity types include ORGANIZATION, PERSON, LOCATION, DATE, TIME, MONEY, and GPE (geo-political entity).
• Chunkers can be constructed using rule-based systems, such as the RegexpParser class provided by NLTK; or using machine learning techniques, such as the ConsecutiveNPChunker presented in this chapter. In either case, part-of-speech tags are often a very important feature when searching for chunks.
• Although chunkers are specialized to create relatively flat data structures, where no two chunks are allowed to overlap, they can be cascaded together to build nested structures.
• Relation extraction can be performed using either rule-based systems which typically look for specific patterns in the text that connect entities and the intervening words; or using machine-learning systems which typically attempt to learn such patterns automatically from a training corpus.