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CS571: Phrase Structure Grammar
The document discusses phrase structure grammar, which is a constituency grammar used in natural language processing. It defines the components of a phrase structure grammar as a set of non-terminals, terminals, a start symbol, and phrase structure rules. It also discusses Chomsky normal form, which restricts rules to be binary branching, and Greibach normal form, where rules start with a terminal symbol. The document provides examples of phrase structure trees for sentences and constructions like passive voice, wh-questions, relative clauses, and coordination.
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CS571: Phrase Structure Grammar
- 1. Phrase Structure Grammar Natural LanguageProcessing Emory University Jinho D. Choi
- 2. 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?
- 3. Context-Free Languages Context-Sensitive Languages MildlyContext-Sensitive Languages Recursively Enumerable Languages Chomsky Hierarchy 3 Regular Languages
- 4. Chomsky Hierarchy • Languagevs.Automata vs. Parsing Complexity 4 Type Language Automata Parsing Complexity 3 Regular Finite state Linear 2 Context-free Pushdown Cubic 1.5 Mildly Context-sensitive Extended pushdown Polynomial 1 Context-sensitive Linear bounded Exponential 0 Recursively Enumerable Turing machine Who cares!
- 5. I bought acaryou 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 ofVBD? ← ?
- 6. I bought acaryou 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 ofVBD 2nd NP is the direct object ofVBD ↳ DT is the determiner of NN Dependent vs. Head Relation?
- 7. 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?
- 8. Chomsky Normal Form 8 “Iwill 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?
- 9. 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?
- 10. Penn Treebank • PennTreebank - 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. 10 ((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
- 11. Passive Construction “I boughta car for you” 11 NP Movement Did NP really move? What about automatic parsing? Is this necessary? 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”
- 12. Exercises • Wh-questions - Whobought 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. 12 • Gapping (bonus) - I bought a car for her, a van for you, and a truck for myself.
- 13. *T*-1 bought acar VBD-NONE- NP VPNP SQ for you PP Who WP WHNP-1 SBARQ Wh-Questions 13 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 this necessary? What about this? Challenging for parsing? Whom did I buy a car for?What did I buy for you?
- 14. Relative Clause 14 I boughtthe 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
- 15. Coordination 15 She wanted thecar 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?
- 16. Gapping Relation 16 I boughta 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 Challenging for parsing?