CS571: Phrase Structure Grammar
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Transcript of CS571: Phrase Structure Grammar
Phrase StructureGrammar
Natural Language ProcessingEmory University
Jinho D. Choi
Phrase Structure Grammar• Phrase structure grammar
- Constituency grammar (e.g., context-free, context-sensitive).
• G = (N, Σ, S, P)
- N : a finite set of non-terminals (word tokens).
- Σ : a finite set of terminals (POS/phrase/clause tags).
- S : a start symbol representing the whole sentence, where S ∈ N.
- P : a finite set of phrase structure rules.
- A ∈ N, α, β ∈ (N ∪ Σ)*, γ ∈ (N ∪ Σ)+
- Context-free : A → α
- Context-sensitive : αA → αγβ
2
Parsing complexity?
Context-Free Languages
Context-Sensitive Languages
Mildly Context-Sensitive Languages
Recursively Enumerable Languages
Chomsky Hierarchy
3
Regular Languages
Chomsky Hierarchy• Language vs. Automata vs. Parsing Complexity
4
Type Language Automata ParsingComplexity
3 Regular Finite state Linear
2 Context-free Pushdown Cubic
1.5 MildlyContext-sensitive
Extended pushdown Polynomial
1 Context-sensitive Linear bounded Exponential
0 Recursively Enumerable
Turing machine Who cares!
I bought a caryou
PRP NNDTVBDPRP
NP
VPNP
S
NP
Phrase Structure Rules• Rule of thumb
- All siblings on the right-hand side should convey meaningful relations.
• “I bought you a car”
5
S → NP VP VP → VBD NP NP → PRP NP
← NP is the subject of (the head of) VP← NP is the object of VBD?← ?
I bought a caryou
PRP NNDTVBDPRP
NP NP
VPNP
S
Phrase Structure Rules• Rule of thumb
- All siblings on the right-hand side should convey meaningful relations.
• “I bought you a car”
6
S → NP VP VP → VBD NP NP NP → DT NN
← NP is the subject of (the head of) VP← 1st NP is the indirect object of VBD 2nd NP is the direct object of VBD
↳ DT is the determiner of NN
Dependent vs. HeadRelation?
Chomsky Normal Form• Chomsky normal form
- All production rules are A → BC or A → α (A, B, C ∈ N, α ∈ Σ).
- How is this useful?
- Should we consider CNF when we design a grammar?
7
CYK parsing
“I will buy a big red car soon later”
Chomsky normal form?
buy a big red car
VBG DT JJ JJ NN
soon
RB
ADVPNP
VP
I will
PRP MD
VPNP
S
Unary rules?
Computationally consistent?
Linguistically sound?
Chomsky Normal Form
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“I will buy a big red car soon later”
buy a big red car
VBG DT JJ JJ NN
soon
RB
NP
NP
I will
PRP MD
VPNP
S
VP ADVP
NP
VP
S → NP VP VP → MD NP VP → VP ADVP VP → VBG NP NP → DT NP|JJ NP|JJ NN
NP → PRP → I MD → will VBG → buy DT → a JJ → big|red NN → car ADVP → RB → soon
Unary rules
Recursive rules?
Greibach Normal Form• Greibach normal form
- The right-hand sides of all production rules start with a terminal.
- How is this useful?
9
Lexicalization
S → I AUXP AUXP → will VP VP → buy DP ADVP DP → a ADJP ADJP → big ADJP ADJP → red NP NP → car ADVP → soon
buy a big red car
NP
soon
ADJP
ADJP
I will
AUXP
RPDP
VP
S
“I will buy a big red car soon later”
Useful?
Penn Treebank• Penn Treebank
- A corpus containing 1M sentences from Wall Street Journal articles.
- Each sentence is parsed as phrase structure trees.
- Each tree is annotated in parenthetical notation.
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((S (NP (PRP I)) (VP (VBD bought) (NP (PRP you)) (NP (DT a) (NN car)))))
I bought a caryou
PRP NNDTVBDPRP
NP NP
VPNP
S
Penn Treebank Tagset
Passive Construction
“I bought a car for you”
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NP MovementDid NP really move?
What aboutautomatic parsing?
Is thisnecessary?
I bought a car
NNDTVBDPRP
NP
VPNP
S
for
IN
you
PRP
PP
NP
A car
NNDT
was bought by me for you
VBD VBN IN PRP IN PRP
NP-1
PPPP
VP
*-1
-NONE-
NP
VP
S
NPNP
“A car was bought by me for you”
Exercises• Wh-questions
- Who bought a car for you?
- What did I buy for you?
- Whom did I buy a car for?
• Relative clause
- I bought the car that she wanted.
- She wanted the car bought by me.
• Coordination
- She wanted the car so I bought it.
- I bought a car and gave it to her.
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• Gapping (bonus)
- I bought a car for her, a van for you, and a truck for myself.
*T*-1 bought a car
VBD-NONE-
NP
VPNP
SQ
for you
PP
Who
WP
WHNP-1
SBARQ
Wh-Questions
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Who bought a car for you?
I buy *T*-1
-NONE-VBPRP
NP
VPNP
VBD
did
SQ
What
WP
WHNP-1
SBARQ
for you
PP
I buy
VBPRP
VPNP
VBD
did
SQ
Whom
WP
WHNP-1
SBARQ
for *T*-1
PP
a car
NP
Is thisnecessary?
Whatabout this?
Challengingfor parsing?
Whom did I buy a car for?What did I buy for you?
Relative Clause
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I bought the car
NNDTVBDPRP
NP
VPNP
S
that she wanted *T*-1
WDT PRP VBD -NONE-
NP
VPNP
SWHNP
SBAR
NP
I bought the car that she wanted She wanted the car bought by me
She wanted the car bought *-1 by me
PRP VBD DT NN VBN -NONE- IN PRP
NP
PPNP
VPNP-1
NP
VPNP
S
Coordination
15
She wanted the car so I bought it.
She wanted the car
VBDPRP
NP
VPNP
S
so
CC
I bought it
VBDPRP
NP
VPNP
S
PRP
S
I bought a car and gave it to her.
I bought a car
VBDPRP
NP
VP
NP
S
and gave it to her
CC VBD PRP
PPNP
VP
VP
Chomsky normal form?
Gapping Relation
16
I bought a car for you , a van for you , and a truck for myself
I bought a car
VBDPRP
NP-1
VP
NP
S
VP
for her
PP-2 NP=1
a van for you a truck for myself
PP=2 NP=1 PP=2
VP VP
Challengingfor parsing?