A Cascaded Finite-State Parser for Syntactic Analysis of Swedish

Proceedings of EACL '99 A Cascaded Finite-State Parser for Syntactic Analysis of Swedish Dimitrios Kokkinakis and Sofie Johansson Kokkinakis Department of Swedish/Spr£kdata Box 200, SE-405 30 G6teborg University, Ghteborg SWEDEN {svedk,svesj}@svenska.gu.se Abstract This report describes the development of a parsing system for written Swedish and is focused on a grammar, the main component of the system, semi- automatically extracted from corpora. A cascaded, finite-state algorithm is ap- plied to the grammar in which the input contains coarse-grained semantic class information, and the output produced reflects not only the syntactic structure of the input, but grammatical functions as well. The grammar has been tested on a variety of random samples of dif- ferent text genres, achieving precision and recall of 94.62% and 91.92% respec- tively, and average crossing rate of 0.04, when evaluated against manually disam- biguated, annotated texts. 1 Introduction This report describes a parsing system for fast and accurate analysis of large bodies of written Swedish. The grammar has been implemented in a modular fashion as finite-state, cascaded machines, henceforth called Cass-SWE, a name adopted from the parser used, Cascaded analy- sis of syntactic structure, (Abney, 1996). Cass- SWE operates on part-of-speech annotated texts and is coupled with a pre-processing mechanism, which distinguishes thousands of phrasal verbs, idioms, and multi-word expressions. Cass-SWE is designed in such a way that semantic informa- tion, inherited by named-entity (NE) identifica- tion software, is taken under consideration; and grammatical functions are extracted heuristically using finite-state transducers. The grammar has been manually acquired from open-source texts by observing legitimately adjacent, part-of-speech chains, and how and which function words sig- nal boundaries between phrasal constituents and clauses. 2 Background 2.1 Cascaded Finite-State Automata Finite-state technology has had a great impact on a variety of Natural Language Processing applica- tions, as well as in industrial and academic Lan- guage Engineering. Attractive properties, such as conceptual simplicity, flexibility, and space and time efficiency, have motivated researchers to cre- ate grammars for natural language using finite- state methods: Koskenniemi et al. (1992); Ap- pelt et al. (1993); Roche (1996); Roche & Schabes (1997). The cascaded, finite-state mechanism we use in this work is described in Abney (1997): "...a finite-state cascade consists of a se- quence of strata, each stratum being de- fined by a set of regular-expression pat- terns for recognizing phrases. [...] The output of stratum 0 consists of parts of speech. The patterns at level l are applied to the output of level I-1 in the manner of a lexicaI analyzer [...] longest match is selected (ties being resolved in favour of the first pattern listed), the matched input symbols are consumed from the in- put, the category of the matched pattern is produced as output, and the cycle re- peats...", (p. 130). 2.2 Swedish Finite-State Grammars There have been few attempts in the past to model Swedish grammars using finite-state methods. K. Church at MIT implemented a Swedish, regular- expression grammar, inspired by ideas from Ejer- hed & Church (1983). Unfortunately, the lexicon and the …

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