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The Simplest NL Applications: Text Searching and Pattern Matching

The Simplest NL Applications: Text Searching and Pattern Matching. Read J & M Chapter 2. Searching for a Single String Using a Nondeterministic FSM. c o c o n u t.

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The Simplest NL Applications: Text Searching and Pattern Matching

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  1. The Simplest NL Applications: Text Searching and Pattern Matching Read J & M Chapter 2

  2. Searching for a Single StringUsing a Nondeterministic FSM c o c o n u t 1 2 3 4 5 6 7 8  

  3. Searching for a Single String Using the Boyer Moore Algorithm

  4. Searching for Multiple Strings  o c o s 2 3 4 5 6 l c o c o n u t 1 2 3 4 5 6 7 8   Example: lococonut

  5. Converting to a Deterministic FSM  o c o s 2 3 4 5 6 l c o c o n u t 1 2 3 4 5 6 7 8  

  6. Regular Expressions • Two different (but related) uses of the term: • Expressions that define all and only the regular languages • (aa ab  ba  bb)* • Expressions in a useful pattern language Matching ip addresses: S!<emphasis> ([0-9]+ (\ . [0-9]+) {3}) </emphasis> ! <inet> $1 </inet>! Finding doubled words: \< ([A-Za-z]+) \s+ \1 \>

  7. REs: Syntax and Semantics Syntax The regular expressions over an alphabet  are all strings over the alphabet  {(, ), , , *} that can be obtained as follows: 1.  and each member of  is a regular expression. 2. If  ,  are regular expressions, then so is . 3. If  ,  are regular expressions, then so is . 4. If  is a regular expression, then so is *. 5. If  is a regular expression, then so is (). 6. Nothing else is a regular expression.

  8. REs: Syntax and Semantics Regular expressions define languages via a semantic interpretation function we'll call L: 1. L() =  and L(a) = {a} for each a  2. If  ,  are regular expressions, then L() = L() L() = all strings that can be formed by concatenating to some string from L() some string from L(). 3. If  ,  are regular expressions, then L() = L()  L() 4. If  is a regular expression, then L(*) = L()* 5. If () is a regular expression, then L( () ) = L() A language is regular if and only if it can be described by a regular expression. Note: Lis compositional.

  9. The Importance of Compositionality What is the meaning of: Mary cooked the yujutes. Mary tyroked the yujutes.

  10. Morphological Analysis • Read J & M Chapter 3 • Recognize words • Parse words

  11. Morphological Parsing Goal: to represent the facts declaratively so that a single representation can be used for both recognition and generation. Note: ^ marks morpheme boundaries. # marks word boundaries.

  12. From Lexical to Intermediate Note: All the transducers in the book are described as lexical:intermediate, but they can run the other direction.

  13. Where Did reg-noun-stem Come From?

  14. We Can Cascade or Compose

  15. From Intermediate to Surface For text, we need spelling rules. x   e / s ^ ___ s # z Read this as “Replace  as e in the context after the /.

  16. Turning the Rule into a Transducer foxes xerox fox#sat

  17. Disambiguation - Local Local ambiguities: # s# asses luxury

  18. Disambiguation - Harder Sometimes additional knowledge is necessary: foxes: fox +N + PL or fox +V +SG Can we think of nouns that cannot also be verbs?

  19. Search • For FSMs, we can build a deterministic machine. • In other cases, we will have to search: • Depth-first • Breadth-first – chart parsing S S VP VP NP PP NP NP V V PR N det N PREP DET N I hit the boy with a bat.

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