Finite-State Methods in Natural Language Processing

1. Finite-State Methods in Natural Language Processing Lauri Karttunen LSA 2005 Summer Institute July 18, 2005

2. Course Outline • July 18: • Intro to computational morphology • XFST • Readings • Lauri Karttunen, “Finite-State Constraints”, The Last Phonological Rule. J. Goldsmith (ed.), pages 173-194, University of Chicago Press, 1993. • Karttunen and Beesley, “25 Years of Finite-State Morphology” • Chapter 1: “Gentle Introduction” (B&K) • July 20: • Regular expressions • More on XFST • Readings • Chapter 2: “Systematic Introduction” • Chapter 3: “The XFST interface”

3. July 25 • Concatenative morphotactics • Constraining non-local dependencies • Readings • Chapter 4. “The LEXC Language” • Chapter 5. “Flag Diacritics” • July 27 • Non-concatenative morphotactics • Reduplication, interdigitation • Readings • Chapter 8. “Non-Concatenative Morphotactics”

4. August 1 • Realizational morphology • Readings • Gregory T. Stump. Inflectional Morphology. A Theory of Paradigm Structure. Cambridge U. Press. 2001. (An excerpt) • Lauri Karttunen, “Computing with Realizational Morphology”, Lecture Notes in Computer Science, Volume 2588, Alexander Gelbukh (ed.), 205-216, Springer Verlag. 2003. • August 3 • Optimality theory • Readings • Paul Kiparsky “Finnish Noun Inflection” Generative Approaches to Finnic and Saami Linguistics, Diane Nelson and Satu Manninen (eds.), pp.109-161, CSLI Publications, 2003. • Nine Elenbaas and René Kager. "Ternary rhythm and the lapse constraint". Phonology 16. 273-329.

5. Getting credit for LSA 207 • There will be three assignments, given on each Wednesday. The first two are to be turned in by the following Monday, the last one by the following Friday. • You will get credit for the course if you solve at least two of the three assignments. The solutions will involve programming in the xfst scripting language. The problems will be easy to solve if you have attended the class. • If you have any problems in doing the assignments, Michael Wagner and I will be happy to help you.

6. Textbook Copies will arrive in the Linguistics Department tomorrow afternoon. You can purchase a copy there tomorrow as soon as the books have arrived. Starting Wednesday, books can Be purchased from our TA, Michael Wagner. The price is $35. With the book comes a software CD for Solaris, Linux, MacOSX and Windows operating systems.

7. LSA 207 Web site • http://lsa.dlp.mit.edu/Class/207 • You can use this username and password to access materials: • Username: LSA207 • Password: seunsehi207 • Your are free to copy, modify and use the slides for whatever purpose provided that you give appropriate credit to the original source. • The readings for Wednesday’s class (“Finite-State Constraints”, “25 Years of Finite-State Morphology” and “Gentle Introduction” (Chapter 1 of B&K book) are posted on the web site).

8. Software • The software on the Book CD dates back to the Spring of 2003. For an update, point your browser to • http://www.stanford.edu/~laurik/.lsa207/ • Please read the README file and the License Agreement before downloading the software. • The updated software supports UTF-8 encoded Unicode input/output. The Book version supports only Latin-1 (ISO-8859-1). • The XFST application will be available locally on some computers (ask Michael). • Check out the web site for the Book: • http://www.fsmbook.com/

9. Finite-State Methods in NLP • Domains of Application • Tokenization • Sentence breaking • Spelling correction • Morphology (analysis/generation) • Phonological disambiguation (Speech Recognition) • Morphological disambiguation (“Tagging”) • Pattern matching (“Named Entity Recognition”) • Shallow Parsing • Types of Finite-State Systems • Classical (non-weighted) automata • Weighted (associated with weights in a semi-ring) • Binary relations (simple transducers) • N-ary relations (multi-tape transducers)

10. Analysis Generation hang V Past leaf N Pl leave N Pl leave V Sg3 leaves hanged hung Computational morphology

11. Two challenges • Morphotactics • Words are composed of smaller elements that must be combined in a certain order: • piti-less-ness is English • piti-ness-less is not English • Phonological alternations • The shape of an element may vary depending on the context • pity is realized as pitiin pitilessness • die becomes dy in dying

12. Morphology is regular (=rational) • The relation between the surface forms of a language and the corresponding lexical forms can be described as a regular relation. • A regular relation consists of ordered pairs of strings. • leaf+N+Pl : leaves hang+V+Past : hung • Any finite collection of such pairs is a regular relation. • Regular relations are closed under operations such as concatenation, iteration, union, and composition. • Complex regular relations can be derived from simple relations.

13. Morphology is finite-state • A regular relation can be defined using the metalanguage of regular expressions. • [{talk} | {walk} | {work}] • [%+Base:0 | %+SgGen3:s | %+Progr:{ing} | %+Past:{ed}]; • A regular expression can be compiled into a finite-state transducer that implements the relation computationally.

14. Finite-state transducer +Base: final state +3rdSg:s a t +Progr:i :n :g a l k w o r +Past:e :d initial state Compilation Regular expression • [{talk} | {walk} | {work}] • [%+Base:0 | %+SgGen3:s | %+Progr:{ing} | %+Past:{ed}];

15. Generation work+3rdSg --> works +Base: +3rdSg:s a:a t:t +Progr:i :n :g a:a l:l k:k w:w o:o r:r +Past:e :d

16. Analysis +Base: +3rdSg:s a:a t:t +Progr:i :n :g a:a l:l k:k w:w o:o r:r +Past:e :d talked --> talk+Past

17. XFST Demo 1 start xfst • xfst[0]: regex • [{talk} | {walk} | {work}] • [% +Base:0 | %+SgGen3:s | %+Progr:{ing} | %+Past:{ed}]; % xfst xfst[0]: compile a regular expression xfst[1]: apply up walked walk+Past apply the result xfst[1]: apply down talk+SgGen3 talks

18. vouloir +IndP +SG + P3 Finite-state transducer veut citation form inflection codes v o u l o i r +IndP +SG +P3 v e u t inflected form Lexical transducer • Bidirectional: generation or analysis • Compact and fast • Comprehensive systems have been built for over 40 languages: • English, German, Dutch, French, Italian, Spanish, Portuguese, Finnish, Russian, Turkish, Japanese, Korean, Basque, Greek, Arabic, Hebrew, Bulgarian, …

19. Morphotactics Lexicon Regular Expression Lexicon FST Lexical Transducer (a single FST) Compiler composition Rules Regular Expressions Rule FSTs Alternations f a t +Adj +Comp t e f a t r How lexical transducers are made

20. fst 1 fst 2 fst n Sequential Model Lexical form Ordered sequence of rewrite rules (Chomsky & Halle ‘68) can be modeled by a cascade of finite-state transducers Johnson ‘72 Kaplan & Kay ‘81 Intermediate form ... Surface form

21. Discovery and Rediscovery • C. Douglas Johnson (1972) showed that • phonological rewrite rules are interpreted in a way that makes them less powerful than they appear • rewrite rules can be modeled by finite transducers • for any two finite transducers applied in a sequence there exists an equivalent single transducer (Schützenberger 1961). • Johnson’s result was ignored and forgotten, rediscovered by Ronald M. Kaplan and Martin Kay at Xerox around 1980.

22. Application constraint • Phonological rewrite rules are not as powerful as they appear because of the constraint that a rule does not apply to its own output. (Johnson 1972, Kaplan&Kay 1980).

23. Sequential application k a N p a n k a m p a n k a m m a n N -> m / _ p p -> m / m _

24. N:m 2 p m N:m ? m 0 ? p 1 N N m p 1 ? m 0 ? p:m Sequential application in detail k a N p a n k a m p a n k a m m a n 0 0 0 2 0 0 0 0 0 0 1 0 0 0

25. 3 0 2 1 Composition N:m p:m k a N p a n k a m m a n N:m m 0 0 0 3 0 0 0 m ? ? p p:m N:m m N ? N N

26. fst n Parallel Model Lexical form ... fst 2 fst 1 Surface form Set of parallel of two-level rules (constraints) compiled into finite-state automata interpreted as transducers Koskenniemi ‘83

27. Koskenniemi 1983 Chomsky&Halle 1968 Lexical form Lexical form rule 1 rule 1 ... rule 2 rule 1 rule n Intermediate form Surface form intersect ... FST rule n Surface form Sequential vs. parallel rules compose

28. Rewrite rules • ? u: ty ? A s • ? u: t I y ? A s • ? u: t u y ? a s • ? o:t u y ? a s Epenthesis Harmony Yawelmani Vowel Harmony Kisseberth 1969 Lowering

29. Two-level constraints ? u:t 0 y ? A s ? o: t u y ? a s Epenthesis: Insert u or i (underspecification) Harmony: Rounding next to a round V of the same height. Lowering: Long u always realized as long o. Underlying representation controls all three alternations.

30. Rewrite Rules vs. Constraints • Two different ways of decomposing the complex relation between lexical and surface forms into a set of simpler relations that can be more easily understood and manipulated. • One approach may be more convenient than the other for particular applications.

31. {a} Language or Relation describes encodes Regular Expression Finite-State Network compiles into a a The Big Picture

32. XFST Demo 2 xfst[0]: define Cat {cat} | {tiger} | {lion}; defined Cat: 640 bytes. 11 states, 12 arcs, 3 paths. ... xfst[0]: xfst[0]: set verbose off xfst[0]: define Dog {dog} | {spaniel} | {poodle}; xfst[0]: regex Cat | Dog ; xfst[1]: apply up apply up> dog dog apply up> panther apply up> apply up> END; xfst[1]: define Animal xfst[0]:

33. xfst[0]: regex Cat & Dog; xfst[1]: print net Sigma: a c d e g i l n o p r s t Size: 13, Label Map: Default Net: Flags: deterministic, pruned, minimized, epsilon_free, ... s0: (no arcs) xfst[1]: xfst[1]: pop xfst[0]: xfst[0]: regex Animal - Dog; xfst[1]: push Cat xfst[2]: test equivalent 1, (0=NO,1=YES) xfst[2]: clear xfst[0]:

34. Network c l e a r e v e f a t h Compiling networks from words xfst[0]: read text clear clever ear ever fat father ^D 432 bytes. 10 states, 12 arcs, 6 paths. read text < file read regex {clear}|{clever}|{ear}|{ever}|{fat}|{father} ;

35. Regular Expression Calculus • Symbols • Simple symbols vs. symbol pairs • Special symbols: ANY, EPSILON • Common regular expression operators • concatenation, union, intersection, negation, composition • Xerox operators • contains, restriction, replacement

36. a Symbols and Labels • Single and multicharacter symbols • a, b, c, … , +Adj, +SG, ^Fin • Special symbols • 0 EPSILON • ?ANY • Symbols vs. symbol pairs • In general, no distinction is made between • a the language {“a”} • a:a the identity relation {<“a”, “a”>}

37. Common RE Operators • concatenation • * +iteration • |union • &intersection* • ~ \ -complementation*, minus* • .x. :crossproduct • .o.composition • * = not applicable to regular relations because the result may not be encodable by a finite-state network.

38. ? b:B c:C a:A [a:A | b:B | c:C | d:D | … ]* d:D Iteration • A* zero or more contatenations of A • A+ one or more concatenations of A • ?* the universal language/ • the universal identity relation

39. a a ? a a ? ? ? Negation • \Aany single symbol that is not in A • \? the null language • ~A any string that is not in A • a • \a • Sigma: a, ? • ~a

40. b:y a:x c:0 Crossproduct • A .x. B The relation that maps every string in A to every string in B, and vice versa • A:B Same as [A .x. B]. a b c .x. x y [a b c] : [x y] {abc}:{xy}

41. b a c b:B a:A c:C b:B c:C a:A d:D Composition • A .o. B The relation C such that if A maps x to y and B maps y to z, C maps x to z. {abc} .o. [a:A | b:B | c:C | d:D]*

42. Xerox RE Operators • $ containment • => restriction • -> @-> replacement • Make it easier to describe complex languages and relations without extending the formal power of finite-state systems.

43. a ? ? a Containment $a [?* a ?*]

44. b a => b _ c b ? a c “Anyamust be preceded byb and followed byc.” ? c c ~[~[?* b] a ?*] & ~[?* a ~[c ?*]] Equivalent expression Restriction