Parsing in NLP Explained

Artificial Intelligence
Natural Language Processing:
Parsing

Rushdi Shams
Computational Linguistics Lab
Western University.
rshams@uwo.ca

Natural Language
• Natural Language means any language we
speak
• We need to process natural language (in
text, speech, etc.) so that machine can
exploit it.
• Applications: numerous!
– Watson (Jeopardy)
– MS Word

Parsing
• The first task for any NLP-based system is to
read (or to parse) the text
• Parsing depends on three components of a
language1. Lexicon
2. Categorization
3. Grammar Rules

Lexicon
stench | breeze | glitter | nothing | wumpus | pit | pits | gold | east | ..
is | see | smell | shoot | feel | stinks | go | grab | carry | kill | turn | …
right | left | east | south | back | smelly | …
here | there | nearby | ahead | right | left | east | south | back | …
me | you | I | it | S=HE | Y’ALL …
John | Mary | Boston | UCB | PAJC | …
the | a | an | …
to | in | on | near | …
and | or | but | …

0|1|2|3|4|5|6|7|8|9

Rushdi Shams, Dept of CSE, KUET,
Bangladesh

4

Grammar Rules
• “The large cat”
• This phrase can be parsed by an NLP-system if
it has a grammar like
Noun Phrase -> Determiner + Adjective + Noun

• If your system finds a phrase or sentence that
has a pattern not mentioned in its set of
Grammar Rules it won’t be able to parse
them.

Therefore...
• Parsing is the process of using grammar
rules to determine whether a sentence is
legal,
• and to obtain its Syntactic Tree

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Syntactic Tree
‘The large cat eats the small rat’

http://www.digitalenema.com/2012_07_01_archive.html

Syntactic Tree

The

large

cat

eats

the

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small

rat

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Syntactic Tree

Article adjective noun

Verb

The

large

cat

eats

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the

small

rat

10

Syntactic Tree


Verb

noun phrase

The

large

cat

eats

Bangladesh

the

small

rat

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Syntactic Tree
Noun phrase


Verb

noun phrase

The

large

cat

eats

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the

small

rat

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Syntactic Tree
Noun phrase

verb phrase


Verb

noun phrase

The

large

cat

eats

the

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small

rat

13

Syntactic Tree
sentence

Noun phrase

verb phrase


Verb

noun phrase

The

large

cat

eats

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the

small

rat

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Label Bracketing
• It is a process of representing the syntactic tree in another way.

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Do yourself: Label Bracket the tree

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Evaluation of Parsing
• The two most frequent and basic measures to
evaluate parsing:

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Precision, Recall, and F1-Score
• The notions are much clearer with a
contingency table-

18

However…

http://www.cafepress.com/barrysworld/1486105

Ambiguity
• There are 2 types of ambiguity1. Lexical Ambiguity: Sentence contains an
idiom/word/term that has more than one
meaning.
Glasses means both drinking glasses and
spectacles

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Ambiguity
2. Structural Ambiguity: Sentence has more
than one syntactic tree
I saw the boy with the telescope

Did you see the boy with a telescope? Or
Did you see the boy who was having a
telescope?
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Structural Ambiguity

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Ambiguity
• Which of the following examples have
lexical ambiguity and which of them carry
structural ambiguity; justify1. The painter put on another coat
2. We like flying planes
3. Visiting relatives can be tiresome

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Ambiguity
• He wrote the note yesterday
• You mean you carried the information by a
bus?
• Connecting wires are tiring in electronics lab
• Squad helps dog bite victim

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Word Sense
• Most of the lexical ambiguity arises from the
differences in word sense.
• Word senses vary due to several factors:
– Synonymy
– Antonymy
– Homonymy
– Polysemy and
– Heteronymy

Synonymy
• Synonyms are different words (or sometimes
phrases) with identical or very similar
meanings.
• Words that are synonyms are said to
be synonymous, and the state of being a
synonym is called synonymy

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Synonymy
•
•
•
•
•

student and pupil (noun)
buy and purchase (verb)
sick and ill (adjective)
quickly and speedily (adverb)
on and upon (preposition)

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Synonymy is a relation between senses
rather than words
• Note that synonyms are defined with respect
to certain senses of words
• pupil as the "aperture in the iris of the eye" is
not synonymous with student.
• Similarly, he expired means the same as he
died, yet my passport has expired cannot be
replaced by my passport has died.

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Synonymy is a relation between senses
rather than words
• Consider the words big and large
• Are they synonyms?:
– How big is the plane?
– Are we travelling with a large or small plane?

• How about?:
– Mrs Benjamin became a big sister of him
– Mrs Benjamin became a large sister of him

Heteronymy
• heteronyms (also known as heterophones)
are words with
– identical spellings (or characters)
– but different pronunciations and meanings.

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Antonymy
• Antonyms are words with opposite or nearly
opposite meanings.
• short and tall
• dead and alive
• increase and decrease

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Homonymy
• A homonym is one of a group of words that
– share the same spelling but
– Have different distinct meaning
• Bank (Financial Institute) vs Bank (Sloping Land)
• Bat (A club for hitting the ball) vs Bat (Mammal)

• Homographs (Bank/Bank, Bat/Bat)
• Homophones (Right/Write, Piece/Peace)

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Polysemy
• Homonymous words that are related with
each other
– The bank was constructed in 1971 (building
related to a financial institute)
– I draw money from the bank (financial institute)

Hypernymy and Hyponymy
• Superclass-subclass structure
– Car is a hypernym of Honda
– Honda is a hyponym of Car

Zeugma Test
• A test to see whether or not two words have
the same sense
– Which flight does serve breakfast?
– Does Lufthansa serve Philadelphia?

• Simply make a conjunction:
– Does Lufthansa serve breakfast and Philadelphia?

WordNet 3.0
• A hierarchically organized lexical database
• On-line thesaurus + aspects of a dictionary
• Some other languages available or under development
– (Arabic, Finnish, German, Portuguese…)

Category

Unique Strings

Noun

117,798

Verb

11,529

Adjective

22,479

Adverb

4,481

Senses of “bass” in Wordnet

WordNet Hypernym Hierarchy for
“bass”

WordNet 3.0
• Where it is:
– http://wordnetweb.princeton.edu/perl/webwn

• Libraries
– Python: WordNet from NLTK
• http://www.nltk.org/Home
– Java:
• JWNL, extJWNL on sourceforge

Difficulties with Natural Language:
Anaphora
• Using pronouns to refer back to entities already
introduced in the text
– After Mary proposed to John, they found a preacher
and got married. For the honeymoon, they went to
Hawaii
– Mary saw a ring through the window and asked John
for it
– Mary threw a rock at the window and broke it

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Indexicality
• Indexical sentences refer to utterance
situation (place, time, etc.)
– I am over here
– Why did you do that?

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Metonymy
• Using one noun phrase to stand for another
– I've read Shakespeare
– Chrysler announced record profits
– The ham sandwich on Table 4 wants another
beer

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Metaphor
• “Non-literal" usage of words and phrases,
often systematic.
– I've tried killing the process but it won't die. Its
parent keeps it alive.

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Summary
• The components of a language
– Lexicon
– Categorization
– Grammar rules

•
•
•
•
•

Syntactic Tree
Label Bracketing
Evaluation of Parsing
Word sense
Problem of Parsing

Parsing in NLP Explained

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