Fall 2005

1

EECS 595 / LING 541 / SI 661
Natural Language Processing

• Fall 2005
• Lecture Notes 4

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Features and unification
3
Introduction

• Grammatical categories have properties
• Constraint-based formalisms
• Example this flights agreement is difficult to
  handle at the level of grammatical categories
• Example many water count/mass nouns
• Sample rule that takes into account features S ?
  NP VP (but only if the number of the NP is equal
  to the number of the VP)

4
Feature structures
CAT NP NUMBER SINGULAR PERSON 3
CAT NP AGREEMENT
NUMBER SG
PERSON 3
Feature paths x agreement number
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Unification

• NUMBER SG NUMBER SG
• NUMBER SG NUMBER PL -
• NUMBER SG NUMBER NUMBER
  SG
• NUMBER SG PERSON 3 ?

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Agreement

• S ? NP VPNP AGREEMENT VP AGREEMENT
• Does this flight serve breakfast?
• Do these flights serve breakfast?
• S ? Aux NP VPAux AGREEMENT NP AGREEMENT

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Agreement

• These flights
• This flight
• NP ? Det NominalDet AGREEMENT Nominal
  AGREEMENT
• Verb ? serveVerb AGREEMENT NUMBER PL
• Verb ? servesVerb AGREEMENT NUMBER SG

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Subcategorization

• VP ? VerbVP HEAD Verb HEADVP HEAD
  SUBCAT INTRANS
• VP ? Verb NPVP HEAD Verb HEADVP HEAD
  SUBCAT TRANS
• VP ? Verb NP NPVP HEAD Verb HEADVP HEAD
  SUBCAT DITRANS

9
Regular Expressions andAutomata
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Regular expressions

• Searching for woodchuck
• Searching for woodchucks with an optional final
  s
• Regular expressions
• Finite-state automata (singular automaton)

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Regular expressions

• Basic regular expression patterns
• Perl-based syntax (slightly different from other
  notations for regular expressions)
• Disjunctions abc
• Ranges A-Z
• Negations Ss
• Optional characters ? and
• Wild cards .
• Anchors and , also \b and \B
• Disjunction, grouping, and precedence

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Writing correct expressions

• Exercise write a Perl regular expression to
  match the English article the

/the//tThe//\btThe\b//a-zA-ZtThea-zA
-Z//(a-zA-Z)tThea-zA-Z/
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A more complex example

• Exercise Write a regular expression that will
  match any PC with more than 500MHz and 32 Gb of
  disk space for less than 1000

/0-9//0-9\.0-90-9//\b0-9(\.0-9
0-9)?\b//\b0-9 (MHzMmegahertzGhz
Ggigahertz)\b//\b0-9 (MbMmegabytes?)\b/
/\b0-9(\.0-9) (GbGgigabytes?)\b/
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Advanced operators
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Substitutions and memory

• Substitutions

s/colour/color/

• Memory (\1, \2, etc. refer back to matches)

s/(0-9)/lt\1gt/
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Eliza Weizenbaum, 1966

• User Men are all alike
• ELIZA IN WHAT WAY
• User Theyre always bugging us about something
  or other
• ELIZA CAN YOU THINK OF A SPECIFIC EXAMPLE?
• User Well, my boyfriend made me come here
• ELIZA YOUR BOYFRIEND MADE YOU COME HERE
• User He says Im depressed much of the time
• ELIZA I AM SORRY TO HEAR THAT YOU ARE DEPRESSED

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Eliza-style regular expressions
Step 1 replace first person references with
second person referencesStep 2 use additional
regular expressions to generate replies Step 3
use scores to rank possible transformations

• s/. YOU ARE (depressedsad) ./I AM SORRY TO
  HEAR YOU ARE \1/
• s/. YOU ARE (depressedsad) ./WHY DO YOU THINK
  YOU ARE \1/
• s/. all ./IN WHAT WAY/
• s/. always ./CAN YOU THINK OF A SPECIFIC
  EXAMPLE/

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Finite-state automata

• Finite-state automata (FSA)
• Regular languages
• Regular expressions

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Finite-state automata (machines)
baa! baaa! baaaa! baaaaa! ...
baa!
a
b
a
a
!
q0
q1
q2
q3
q4
finalstate
state
transition
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Input tape
q0
a
b
a
!
b
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Finite-state automata

• Q a finite set of N states q0, q1, qN
• ? a finite input alphabet of symbols
• q0 the start state
• F the set of final states
• ?(q,i) transition function

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State-transition tables
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The FSM toolkit and friends

• Developed at ATT Research (Riley, Pereira,
  Mohri, Sproat)
• Download http//www.research.att.com/sw/tools/fs
  m/tech.htmlhttp//www.research.att.com/sw/tools/l
  extools/
• Tutorial available
• 4 useful parts FSM, Lextools, GRM, Dot
  (separate)
• /data2/tools/fsm-3.6/bin
• /data2/tools/lextools/bin
• /data2/tools/dot/bin

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D-RECOGNIZE
function D-RECOGNIZE (tape, machine) returns
accept or reject index ? Beginning of tape
current-state ? Initial state of machine loop
if End of input has been reached then
if current-state is an accept state then
return accept else return
reject elsif transition-table
current-state, tapeindex is empty then
return reject else current-state ?
transition-table current-state, tapeindex
index ? index 1end
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Adding a failing state
a
b
a
a
!
q0
q1
q2
q3
q4
!
!
b
!
b
!
b
b
a
qF
a
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Languages and automata

• Formal languages regular languages, non-regular
  languages
• deterministic vs. non-deterministic FSAs
• Epsilon (?) transitions

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Using NFSAs to accept strings

• Backup add markers at choice points, then
  possibly revisit underexplored markers
• Look-ahead look ahead in input
• Parallelism look at alternatives in parallel

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Using NFSAs
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More about FSAs

• Transducers
• Equivalence of DFSAs and NFSAs
• Recognition as search depth-first,
  breadth-search

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Recognition using NFSAs
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Regular languages

• Operations on regular languages and FSAs
  concatenation, closure, union
• Properties of regular languages (closed under
  concatenation, union, disjunction, intersection,
  difference, complementation, reversal, Kleene
  closure)

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An exercise

• JM 2.8. Write a regular expression for the
  language accepted by the NFSA in the Figure.

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Morphology and Finite-State Transducers
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Morphemes

• Stems, affixes
• Affixes prefixes, suffixes, infixes hingi
  (borrow) humingi (agent) in Tagalog,
  circumfixes sagen gesagt in German
• Concatenative morphology
• Templatic morphology (Semitic languages)
• lmd (learn), lamad (he studied), limed (he
  taught), lumad (he was taught)

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Morphological analysis

• rewrites
• unbelievably

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Inflectional morphology

• Tense, number, person, mood, aspect
• Five verb forms in English
• 40 forms in French
• Six cases in Russianhttp//www.departments.buckn
  ell.edu/russian/language/case.html
• Up to 40,000 forms in Turkish (you cause X to
  cause Y to do Z)

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Derivational morphology

• Nominalization computerization, appointee,
  killer, fuzziness
• Formation of adjectives computational,
  embraceable, clueless

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Finite-state morphological parsing

• Cats cat N PL
• Cat cat N SG
• Cities city N PL
• Geese goose N PL
• Ducks (duck N PL) or (duck V 3SG)
• Merging V PRES-PART
• Caught (catch V PAST-PART) or (catch V PAST)

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Principles of morphological parsing

• Lexicon
• Morphotactics (e.g., plural follows noun)
• Orthography (easy ? easier)
• Irregular nouns e.g., geese, sheep, mice
• Irregular verbs e.g., caught, ate, eaten

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FSA for adjectives

• Big, bigger, biggest
• Cool, cooler, coolest, coolly
• Red, redder, reddest
• Clear, clearer, clearest, clearly, unclear,
  unclearly
• Happy, happier, happiest, happily
• Unhappy, unhappier, unhappiest, unhappily
• What about unbig, redly, and realest?

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Using FSA for recognition

• Is a string a legitimate word or not?
• Two-level morphology lexical level surface
  level (Koskenniemi 83)
• Finite-state transducers (FST) used for regular
  relations
• Inversion and composition of FST

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Orthographic rules

• Beg/begging
• Make/making
• Watch/watches
• Try/tries
• Panic/panicked

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Combining FST lexicon and rules

• Cascades of transducersthe output of one
  becomes the input of another

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Weighted Automata
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Phonetic symbols

• IPA
• Arpabet
• Examples

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Using WFST for language modeling

• Phonetic representation
• Part-of-speech tagging

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Word Classes andPart Of Speech Tagging
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Some POS statistics

• Preposition list from COBUILD
• Single-word particles
• Conjunctions
• Pronouns
• Modal verbs

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Tagsets for English

• Penn Treebank
• Other tagsets (see Week 1 slides)

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POS ambiguity

• Degrees of ambiguity (DeRose 1988)
• Rule-based POS tagging
• ENGTWOL (Voutilainen et al. )
• Sample rule
• Adverbial-That rule (it isnt that
  odd) (Given input thatif (1
  A/ADV/QUANT) (2 SENT-LIM) (NOT 1
  SVOC/A) (not a verb like consider)then
  eliminate non-ADV tagselse eliminate ADV tag

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Evaluating POS taggers

• Percent correct
• What is the lower bound on a systems
  performance?
• What about the upper bound?

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Kappa

• N number of items (index i)
• n number of categories (index j)
• k number of annotators
• when k gt .8 agreement is considered high

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Midterm reading list

• Chapter 1 Introduction
• Chapter 2 Regular expressions and automata
• Chapter 3 Morphology and finite-state
  transducers FSM tutorial
• Chapter 8 Word classes and POS tagging
• Chapter 9 Context-free grammars for English
• Chapter 10 Parsing with context-free grammars
• Chapter 11 - Features and unification

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Syntaxscape

• Written by Juno Suk of Lucent
• http//www.cs.columbia.edu/radev/syntaxscape/

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(No Transcript)
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Read by yourselves

• 9.9. Spoken language syntax
• 9.10. Grammar equivalence
• 9.11. Finite-state and context-free grammars