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A novel approach to text classification
Umeå University, Faculty of Science and Technology, Department of Computing Science. (Foundations of Language Processing)
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis explores the foundations of text classification, using both empirical and deductive methods, with a focus on author identification and syntactic methods. We strive for a thorough theoretical understanding of what affects the effectiveness of classification in general. 

To begin with, we systematically investigate the effects of some parameters on the accuracy of author identification. How is the accuracy affected by the number of candidate authors, and the amount of data per candidate? Are there differences in how methods react to the changes in parameters? Using the same techniques, we see indications that methods previously thought to be topic-independent might not be so, but that syntactic methods may be the best option for avoiding topic dependence. This means that previous studies may have overestimated the power of lexical methods. We also briefly look for ways of spotting which particular features might be the most effective for classification. Apart from author identification, we apply similar methods to identifying properties of the author, including age and gender, and attempt to estimate the number of distinct authors in a text sample. In all cases, the techniques are proven viable if not overwhelmingly accurate, and we see that lexical and syntactic methods give very similar results. 

In the final parts, we see some results of automata theory that can be of use for syntactic analysis and classification. First, we generalise a known algorithm for finding a list of the best-ranked strings according to a weighted automaton, to doing the same with trees and a tree automaton. This result can be of use for speeding up parsing, which often runs in several steps, where each step needs several trees from the previous as input. Second, we use a compressed version of deterministic finite automata, known as failure automata, and prove that finding the optimal compression is NP-complete, but that there are efficient algorithms for finding good approximations. Third, we find and prove the derivatives of regular expressions with cuts. Derivatives are an operation on expressions to calculate the remaining expression after reading a given symbol, and cuts are an extension to regular expressions found in many programming languages. Together, these findings may be able to improve on the syntactic analysis which we have seen is a valuable tool for text classification.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2017. , p. 176
Series
Report / UMINF, ISSN 0348-0542 ; 17.16
Keywords [en]
Text classification, natural language processing, automata
National Category
Language Technology (Computational Linguistics) Computer Sciences
Research subject
Computer Science
Identifiers
URN: urn:nbn:se:umu:diva-138917ISBN: 978-91-7601-740-1 (print)OAI: oai:DiVA.org:umu-138917DiVA, id: diva2:1137971
Public defence
2017-09-29, N430, Naturvetarhuset, Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2017-09-04 Created: 2017-09-03 Last updated: 2018-06-09Bibliographically approved
List of papers
1. Efficient enumeration of weighted tree languages over the tropical semiring
Open this publication in new window or tab >>Efficient enumeration of weighted tree languages over the tropical semiring
2019 (English)In: Journal of computer and system sciences (Print), ISSN 0022-0000, E-ISSN 1090-2724, Vol. 104, p. 78p. 119-130Article in journal (Refereed) Published
Abstract [en]

We generalise a search algorithm by Mohri and Riley from strings to trees. The original algorithm takes as input a nondeterministic weighted automaton M over the tropical semiring and an integer N, and outputs N strings of minimal weight with respect to M. In our setting, M is a weighted tree automaton, again over the tropical semiring, and the output is a set of N trees with minimal weight in this language. We prove that the algorithm is correct, and that its time complexity is a low polynomial in N and the relevant size parameters of M. 

Publisher
p. 78
Keywords
weighted tree automaton, N-best analysis, tropical semiring
National Category
Computer Sciences
Research subject
Computer Science
Identifiers
urn:nbn:se:umu:diva-132963 (URN)10.1016/j.jcss.2017.03.006 (DOI)000472246300008 ()
Available from: 2017-03-27 Created: 2017-03-27 Last updated: 2019-07-12Bibliographically approved
2. Compression of finite-state automata through failure transitions
Open this publication in new window or tab >>Compression of finite-state automata through failure transitions
2014 (English)In: Theoretical Computer Science, ISSN 0304-3975, E-ISSN 1879-2294, Vol. 557, p. 87-100Article in journal (Refereed) Published
Abstract [en]

Several linear-time algorithms for automata-based pattern matching rely on failure transitions for efficient back-tracking. Like epsilon transitions, failure transition do not consume input symbols, but unlike them, they may only be taken when no other transition is applicable. At a semantic level, this conveniently models catch-all clauses and allows for compact language representation.

This work investigates the transition-reduction problem for deterministic finite-state automata (DFA). The input is a DFA A and an integer k. The question is whether k or more transitions can be saved by replacing regular transitions with failure transitions. We show that while the problem is NP-complete, there are approximation techniques and heuristics that mitigate the computational complexity. We conclude by demonstrating the computational difficulty of two related minimisation problems, thereby cancelling the ongoing search for efficient algorithms.

Place, publisher, year, edition, pages
Elsevier, 2014
Keywords
failure automata, pattern matching, automata minimisation
National Category
Computer Sciences
Research subject
Computer Science
Identifiers
urn:nbn:se:umu:diva-93329 (URN)10.1016/j.tcs.2014.09.007 (DOI)000343784800008 ()
Funder
Swedish Research Council, 621-2011-6080
Available from: 2014-09-17 Created: 2014-09-17 Last updated: 2018-06-07Bibliographically approved
3. Derivatives of regular expressions with cuts
Open this publication in new window or tab >>Derivatives of regular expressions with cuts
2017 (English)Report (Other academic)
Abstract [en]

Derivatives of regular expressions are an operation which for a given expression produces an expression for what remains after a specific symbol has been read. This can be used as a step in transforming an expression into a finite string automaton. Cuts are an extension of the ordinary regular expressions; the cut operator is essentially a concatenation without backtracking, formalising a behaviour found in many programming languages. Just as for concatenation, we can also define an iterated cut operator. We show and derive expressions for the derivatives of regular expressions with cuts and iterated cuts.

Publisher
p. 7
Series
Report / UMINF, ISSN 0348-0542 ; 17.03
Keywords
regular expression, derivative, cut expression
National Category
Language Technology (Computational Linguistics)
Research subject
computational linguistics
Identifiers
urn:nbn:se:umu:diva-138916 (URN)
Available from: 2017-09-03 Created: 2017-09-03 Last updated: 2018-06-09Bibliographically approved

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