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Séminaire Algo - Liat Peterfreund
Séminaire Algo - Liat Peterfreund
9-Nov-2021 14:00
Age: 23 days





Liat Peterfreund

Grammars for Document Spanners

Seminar room 4B125, Copernic building

Abstract: We propose a new grammar-based language for defining information-extractors from documents (text) that is built upon the well-studied framework of Document Spanners for extracting structured data from text. While previously studied formalisms for document spanners are mainly based on regular expressions, we use an extension of context-free grammars, called extraction grammars, to define the new class of context-free spanners. Extraction grammars are simply context-free grammars extended with variables that  capture interval positions of the document, namely spans. While regular expressions are efficient for tokenizing and tagging, context-free grammars are also efficient for capturing structural properties. Indeed, we show that context-free spanners are strictly more expressive than their regular counterparts. We reason about the expressive power of our new class and present a pushdown-automata model that captures it. We show that extraction grammars can be evaluated with polynomial data complexity. Nevertheless, as the degree of the polynomial depends on the query, we present an enumeration algorithm for (a subset of) extraction grammars that, after quintic preprocessing, outputs the results sequentially, without repetitions, with a constant delay between every two consecutive ones.

This talk is based on a paper from ICDT21.








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