The Simplest NL Applications: Text Searching and Pattern Matching

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The Simplest NL Applications Text Searching and
Pattern Matching
Read J M Chapter 2
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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
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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
?
?
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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!ltemphasisgt (0-9 (\ .
0-9) 3) lt/emphasisgt ! ltinetgt 1
lt/inetgt! Finding doubled words \lt (A-Za-z)
\s \1 \gt
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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.
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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 L is
compositional.
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The Importance of Compositionality
What is the meaning of Mary cooked the
yujutes. Mary tyroked the yujutes.
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Morphological Analysis

• Read J M Chapter 3
• Recognize words

• Parse words

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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.
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From Lexical to Intermediate
Note All the transducers in the book are
described as lexicalintermediate, but they can
run the other direction.
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Where Did reg-noun-stem Come From?
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We Can Cascade or Compose
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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 /.
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Turning the Rule into a Transducer
foxes xerox foxsat
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Disambiguation - Local
Local ambiguities
s
asses
luxury
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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?
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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.