On Lookaheads in Regular Expressions with Backreferences

Nariyoshi Chida*, Tachio Terauchi

*この研究の対応する著者

研究成果: Conference contribution

抄録

Many modern regular expression engines employ various extensions to give more expressive support for real-world usages. Among the major extensions employed by many of the modern regular expression engines are backreferences and lookaheads. A question of interest about these extended regular expressions is their expressive power. Previous works have shown that (i) the extension by lookaheads does not enhance the expressive power, i.e., the expressive power of regular expressions with lookaheads is still regular, and that (ii) the extension by backreferences enhances the expressive power, i.e., the expressive power of regular expressions with backreferences (abbreviated as rewb) is no longer regular. This raises the following natural question: Does the extension of regular expressions with backreferences by lookaheads enhance the expressive power of regular expressions with backreferences? This paper answers the question positively by proving that adding either positive lookaheads or negative lookaheads increases the expressive power of rewb (the former abbreviated as rewblp and the latter as rewbln). A consequence of our result is that neither the class of finite state automata nor that of memory automata (MFA) of Schmid [14] (which corresponds to regular expressions with backreferenes but without lookaheads) corresponds to rewblp or rewbln. To fill the void, as a first step toward building such automata, we propose a new class of automata called memory automata with positive lookaheads (PLMFA) that corresponds to rewblp. The key idea of PLMFA is to extend MFA with a new kind of memories, called positive-lookahead memory, that is used to simulate the backtracking behavior of positive lookaheads. Interestingly, our positive-lookahead memories are almost perfectly symmetric to the capturing-group memories of MFA. Therefore, our PLMFA can be seen as a natural extension of MFA that can be obtained independently of its original intended purpose of simulating rewblp.

本文言語English
ホスト出版物のタイトル7th International Conference on Formal Structures for Computation and Deduction, FSCD 2022
編集者Amy P. Felty
出版社Schloss Dagstuhl- Leibniz-Zentrum fur Informatik GmbH, Dagstuhl Publishing
ISBN(電子版)9783959772334
DOI
出版ステータスPublished - 2022 6月 1
イベント7th International Conference on Formal Structures for Computation and Deduction, FSCD 2022 - Haifa, Israel
継続期間: 2022 8月 22022 8月 5

出版物シリーズ

名前Leibniz International Proceedings in Informatics, LIPIcs
228
ISSN(印刷版)1868-8969

Conference

Conference7th International Conference on Formal Structures for Computation and Deduction, FSCD 2022
国/地域Israel
CityHaifa
Period22/8/222/8/5

ASJC Scopus subject areas

  • ソフトウェア

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