Partial Parsing

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????? ???? ?????? - ????? ???Partial Parsing

• ???? ??????
• ?????? ????? ?????
• ?????????? ?? ????

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Syntax

• The study of grammatical relations between words
  and other units within the sentence.
  The Concise Oxford Dictionary of Linguistics
• the way in which linguistic elements (as words)
  are put together to form constituents (as phrases
  or clauses) Merriam-Webster Dictionary

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Brackets

• I prefer a morning flight
• S NP pro IVP V preferNP Det a Nom N
  morning N flight

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Parse Tree
S
VP
NP
NP
Nom
Noun
Noun
Det
Verb
Pronoun
I
prefer
a
morning
flight
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Parsing

• The problem of mapping from a string of words to
  to its parse tree is called parsing.

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Generative Grammar

• A set of rules which indicate precisely what can
  be and cannot be a sentence in a language.
• A grammar which precisely specifies the
  membership of the set of all the grammatical
  sentences in the language in question and
  therefore excludes all the ungrammatical
  sentences.

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Formal Languages

• The set of all grammatical sentences in a given
  natural language.
• Are natural languages regular?

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English is not a regular language!

• anbn is not regular
• Look at the following English sentences
• John and Mary like to eat and sleep,
  respectively.
• John, Mary, and Sue like to eat, sleep, and
  dance, respectively.
• John, Mary, Sue, and Bob like to eat, sleep,
  dance, and cook, respectively.

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Constituents

• Certain groupings of words behave as
  constituents.
• Constituents are able to occur in various
  sentence positions
• ????? ?? ???? ????
• ????? ???? ???? ?? ???? ????
• ???? ???? ?? ????

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The Noun Phrase (NP)

• Examples
• He
• Ariel Sharon
• The prime minister
• The minister of defense during the war in
  Lebanon.
• They can all appear in a similar context___ was
  born in Kfar-Malal

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Prepositional Phrases

• Examples
• the man in the white suit
• Come and look at my paintings
• Are you fond of animals?
• Put that thing on the floor

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Verb Phrases

• Examples
• Getting to school on time was a struggle.
• He was trying to keep his temper.
• That woman quickly showed me the way to hide.

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Chunking

• Text chunking is dividing sentences into
  non-overlapping phrases.
• Noun phrase chunking deals with extracting the
  noun phrases from a sentence.
• While NP chunking is much simpler than parsing,
  it is still a challenging task to build a
  accurate and very efficient NP chunker.

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What is it good for

• The importance of chunking derives from the fact
  that it is used in many applications
• Information Retrieval Question Answering
• Machine Translation
• Preprocessing before full syntactic analysis
• Text to speech
• Many other Applications

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What kind of structures should a partial parser
identify?

• Different structures useful for different tasks
• Partial constituent structureNPI VPsaw NPa
  tall man in the park.
• Prosodic segmentsI saw a tall man in the
  park.
• Content word groupsI saw a tall man in
  the park.

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Chunk Parsing

• Goal divide a sentence into a sequence of
  chunks.
• Chunks are non-overlapping regions of a text
• I saw a tall man in the park.
• Chunks are non-recursive
• a chunk can not contain other chunks
• Chunks are non-exhaustive
• not all words are included in chunks

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Chunk Parsing Examples

• Noun-phrase chunking
• I saw a tall man in the park.
• Verb-phrase chunking
• The man who was in the park saw me.
• Prosodic chunking
• I saw a tall man in the park.

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Chunks and Constituency

• Constituents a tall man in the park.
• Chunks a tall man in the park.
• Chunks are not constituents
• Constituents are recursive
• Chunks are typically subsequences of Constituents
• Chunks do not cross constituent boundaries

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Chunk Parsing Accuracy

• Chunk parsing achieves higher accuracy
• Smaller solution space
• Less word-order flexibility within chunks than
  between chunks
• Better locality
• Fewer long-range dependencies
• Less context dependence
• No need to resolve ambiguity
• Less error propagation

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Chunk Parsing Domain Specificity

• Chunk parsing is less domain specific
• Dependencies on lexical/semantic information tend
  to occur at levels "higher" than chunks
• Attachment
• Argument selection
• Movement
• Fewer stylistic differences within chunks

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Chunk Parsing Efficiency

• Chunk parsing is more efficient
• Smaller solution space
• Relevant context is small and local
• Chunks are non-recursive
• Chunk parsing can be implemented with a finite
  state machine

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Psycholinguistic Motivations

• Chunk parsing is psycholinguistically motivated
• Chunks as processing units
• Humans tend to read texts one chunk at a time
• Eye-movement tracking studies
• Chunks are phonologically marked
• Pauses, Stress patterns
• Chunking might be a first step in full parsing

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Chunk Parsing Techniques

• Chunk parsers usually ignore lexical content
• Only need to look at part-of-speech tags
• Techniques for implementing chunk parsing
• Regular expression matching / Finite State
  Machines
• Transformation Based Learning
• Memory Based Learning
• Others

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Regular Expression Matching

• Define a regular expression that matches the
  sequences of tags in a chunk
• A simple noun phrase chunk regexpltDTgt? ltJJgt
  ltNN.?gt
• Chunk all matching subsequencesthe/DT little/JJ
  cat/NN sat/VBD on/IN the/DT mat/NNthe/DT
  little/JJ cat/NN sat/VBD on/IN the/DT mat/NN
• If matching subsequences overlap, the first one
  gets priority

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Chunking as Tagging

• Map Part of Speech tag sequences to I,O,B
• I tag is part of an NP chunk
• O tag is not part of
• B the first tag of an NP chunk which
  immediately follows another NP chunk
• Example
• Input The little cat sat on the mat
• Output B I I O O B I

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Chunking State of the Art

• Depending on task specification and test set
  90-95

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Homework
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Context Free Grammars

• Putting the constituents together
• Next Week