Contributions to the Theory of Finite-State Based Grammars

This dissertation is a theoretical study of finite-state based grammars used in natural language processing. The study is concerned with certain varieties of finite-state intersection grammars (FSIG) whose parsers define regular relations between surface strings and annotated surface strings. The study focuses on the following three aspects of FSIGs: (i) Computational complexity of grammars under limiting parameters In the study, the computational complexity in practical natural language processing is approached through performance-motivated parameters on structural complexity. Each parameter splits some grammars in the Chomsky hierarchy into an infinite set of subset approximations. When the approximations are regular, they seem to fall into the logarithmic-time hierarchyand the dot-depth hierarchy of star-free regular languages. This theoretical result is important and possibly relevant to grammar induction. (ii) Linguistically applicable structural representations Related to the linguistically applicable representations of syntactic entities, the study contains new bracketing schemes that cope with dependency links, left- and right branching, crossing dependencies and spurious ambiguity. New grammar representations that resemble the Chomsky-Schützenberger representation of context-free languages are presented in the study, and they include, in particular, representations for mildly context-sensitive non-projective dependency grammars whose performance-motivated approximations are linear time parseable. (iii) Compilation and simplification of linguistic constraints Efficient compilation methods for certain regular operations such as generalized restriction are presented. These include an elegant algorithm that has already been adopted as the approach in a proprietary finite-state tool. In addition to the compilation methods, an approach to on-the-fly simplifications of finite-state representations for parse forests is sketched. These findings are tightly coupled with each other under the theme of locality. I argue that the findings help us to develop better, linguistically oriented formalisms for finite-state parsing and to develop more efficient parsers for natural language processing. Avainsanat: syntactic parsing, finite-state automata, dependency grammar, first-order logic, linguistic performance, star-free regular approximations, mildly context-sensitive grammar

A computational model of modern standard arabic verbal morphology based on generation

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Filosofía y Letras, Departamento de Lingüística, Lenguas Modernas, Lógica y Fª de la Ciencia y Tª de la Literatura y Literataura Comparada. Fecha de lectura: 29-01-2013The computational handling of non-concatenative morphologies is still a challenge in the field of natural language processing. Amongst the various areas of research, Arabic morphology stands out due to its highly complex structure. We propose a model for Arabic verbal morphology based on a root-and-pattern approach, which satisfies both computational consistency and an elegant formalization. Our model defines an abstract representation of prosodic templates and a set of intertwined morphemes that operate at different phonological levels, as well as a separate module of rewrite rules to deal with morphophonological and orthographic alterations. Our verbal system model asserts that Arabic exhibits two conjugational classes. The computational system, named Jabalín, is focused on generation—the program generates a full annotated lexicon of verbal forms, which is subsequently used to develop a morphological analyzer and generator. The input of the system consists of a lexicon of 15,452 verb lemmas of both Classical Arabic and Modern Standard Arabic—taken from El-Dahdah (1991)—comprising a total of 3,706 roots. The output of the system is a lexicon of 1,684,268 verbal inflected forms. We carried out an evaluation against a lexicon of inflected verbs provided by the analyzer ElixirFM (Smrž, 2007a; 2007b), which we considered a Golden Standard, achieving a precision of 99.52%. Additionally, we compared our lexicon with a list of the most frequent verb lemmas—including the most frequent verbs from each conjugation—taken from Buckwalter and Parkinson (2010). The list includes 825 verbs which are all included in our lexicon and passed an evaluation test with 99.27% of accuracy. Jabalín is available under a GNU license, and can be accessed and tested through an online interface, at http://elvira.lllf.uam.es/jabalin/, hosted at the LLI-UAM lab. The Jabalín interface provides different functionalities: analyze a form, generate the inflectional paradigm of a verb lemma, derive a root, show quantitative data, and explore the database, which includes data from the evaluation. ii Key words: Computational Linguistics, Natural Language Processing, Arabic Computational Morphology, Root-and-Pattern Morphology, Non-concatenative Morphology, Templatic Morphology, Root-and-Prosody Morphology, Computational Prosodic Morphology.Los sistemas morfológicos de tipo no concatenativo siguen siendo uno de los mayores retos para el procesamiento del lenguaje natural. Entre las diversas líneas de investigación, el estudio de la morfología del árabe destaca por ser un sistema de gran complejidad estructural. En el presente proyecto de investigación, se propone un modelo de morfología verbal del árabe basado en un enfoque root-and-pattern, así como formalmente elegante y coherente desde el punto de vista computacional. El modelo propuesto se apoya fundamentalmente en una formalización abstracta de los esquemas prosódicos y su interrelación con el material morfológico. Paralelamente, el sistema cuenta con un módulo de reglas que tratan las alteraciones morfofonológicas y ortográficas del árabe. El modelo del sistema verbal propone, y se asienta en la idea de que, existen sólo dos clases conjugacionales en árabe. El sistema computacional, llamado Jabalín, está orientado a la generación: el programa genera un lexicón de formas verbales con la información lingüística asociada. El lexicón se emplea a continuación para desarrollar un analizador y generador morfológicos. Como entrada, el sistema recibe un lexicón de lemas verbales de 15.452 entradas (tomado de El-Dahdah, 1991), que combina léxico tanto del árabe clásico como del árabe estándar moderno, y cuenta con un total de 3.706 raíces. La salida es un lexicón de 1.684.268 formas verbales flexionadas. Se ha llevado a cabo una evaluación contra un lexicón de formas verbales extraído del analizador ElixirFM (Smrž, 2007a; 2007b), con una precisión de 99,52%. Por otro lado, el lexicón se ha evaluado también contra una lista de verbos más frecuentes (incluyendo los lemas más frecuentes de cada tipo de conjugación) sacada de Buckwalter y Parkinson (2010). El total de los 825 verbos que componen la lista están incluidos en nuestro lexicón de lemas verbales y presentan una precisión del 99.27%. El sistema Jabalín, desarrollado bajo licencia GNU, cuenta además con una interfaz web donde se pueden realizar consultas en árabe, http://elvira.lllf.uam.es/jabalin/, albergada en el LLI-UAM. La interfaz cuenta iv con varias funcionalidades: analizar forma, generar flexión de un lema verbal, derivar raíz, mostrar datos cuantitativos, y explorar la base de datos, que incluye los datos de la evaluación. Palabras clave: Lingüística Computacional, Procesamiento del Lenguaje Natural, Morfología Computacional del Árabe, morfología root-and-pattern, morfología no-concatenativa, morfología templática, morfología root-and-prosody, morfología prosódica computacional