PAT expressions: an algebra for text search

Text search operations are used to locate and retrieve needed information from some text collection. In traditional information retrieval, text search is a means for identifying relevant documents [Salton83, Lee85]. By specifying selection criteria for the text of a document, the reader can choose a subset of a given set of documents. If the text collection is defined not as a set of documents, but more generally as a structure containing some parts, then text search involves the specification of those parts of interest to the reader. The structure of the documents may be determined by the search system, by the author, by the text installer, or by the reader. In the PAT system [Gonnet87a, Fawcett89a, Fawcett89b] text search operations are expressions that efficiently combine traditional search capabilities with some new, powerful facilities. PAT contains means for lexical search, proximity search, contextual search and Boolean search [Hollaar79, Larson84, Lee85, Burkowski91]. It also contains more rare operation types, including position and frequency search. Furthermore, a novel feature in PAT is the capability by which a reader can define structures for a text and use these structures in subsequent operations. One of the goals of this paper is to introduce the powerful search capabilities of PAT expressions.