101035 中文信息处理

Chinese NLP

Lecture 8

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句——语法分析（ 1 ）Grammatical Analysis (1)

• 语法分析基础（ Basics)

• 形式语法（ Formal grammars)

• 上下文无关语法（ Context-free grammars ）• 依存语法（ Dependency grammar ）

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语法分析基础Basics

• Constituency （句子成分）• Grammar, or strictly speaking syntax, is about how

words are put together to make sentences.

• A constituent is a group of words, assuming a certain syntactic role.

• A constituent stands in certain grammatical relations to other constituents.

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• Examples of Constituents

• English noun phrases

• English noun phrases appear in similar syntactic environments.

• But an individual word in a noun phrase cannot.

Harry the Horse, a high-class spot such as Mindy’sthe Broadway coppers, the reason he comes into the Hot Box

a high-class spot such as Mindy’s attracts. . .the Broadway coppers love. . .

* a high-class attracts. . .* the love. . .

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• Examples of Constituents

• Chinese phrases

• “ 把……”，“被……”

• Structural account

老师被迟到的学生逗乐了。 = 迟到的学生把老师逗乐了。 ≠ * 老师被迟到的学生被逗乐了。老师被冤枉的事情传开了。≠ * 冤枉的事情把老师传开了。 = 老师被冤枉的事情被传开了。电话被监听的老师找到了。 = 监听的老师把电话找到了。 = 电话被监听的老师被找到了。

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形式语法Formal Grammars

• Enumeration

• The grammar of a language can be the set of all enumerated sentences.

• We cannot exhaust all possible sentences or deal with new sentences.

• Rather, we should use recursive language to describe sentences with internal structure.

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• Regular expressions

• Symbols of a language (POS)

ART （冠词） , PRON （代词）N （名词） , V （动词） , ADJ （形容词） , ADV （副词）

• Combination patterns of the symbols

ART+N ； ART+N+V ； ART+ADJ+N+V

• Regular expression symbols

• *: occurs zero or more times

ART+ADJ*+N

• +: occurs 1 or more times

ART+ADJ++N

• ( ): occurs zero or 1 time

ART+(ADJ)+N

• |: disjunctions

N | PRON + V

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In-Class Exercise

• Write a regular expression that can describe all the following phrases.

老张是一个环卫工老张是一个聪明的环卫工。老张是一个聪明勤劳的环卫工。他是一个聪明的人。

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• Rules in a Formal Grammar

• A set of rules or productions express the ways that symbols of the language can be grouped and ordered together.

• S （句子） , NP （名词短语） , VP （动词短语） , PP （介词短语）

• Formal Definition of a Formal Grammar

• N: a set of non-terminal symbols (or variables)

• Σ: a set of terminal symbols (disjoint from N)

• R: a set of rules or productions, each of the form A β, where A is a nonterminal, β is a string of symbols from the infinite set of strings (Σ ∪N)∗

• S: a designated start symbol

S NP VP, NP Det N,VP V NP, PP Prep NP

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上下文无关语法Context-Free Grammars

• Definition

• As a kind of formal grammar, Context-Free Grammars (CFGs) are the most commonly used mathematical system for modeling the constituent structure of a language. They are also called Phrase-Structure Grammars.

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• Parse tree

• A parse tree is a tree structure that shows how the rules in a CFG are used in a sequence to expand a non-terminal node into terminal nodes.

NP → Det Nominal

Det → aNominal →

Noun

Noun → flight

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• An English Example

• Lexicon

I prefer a morning flight.

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• An English Example

• Grammar

I prefer a morning flight.

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• An English Example

• Parse Tree

I prefer a morning flight.

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• Chinese Examples

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• Treebanks

• A Treebank is a corpus in which every sentence is syntactically annotated with a parse tree.

• Treebanks are invaluable resources for NLP, especially parsing.

• The Penn Treebank Project is a representative treebank.

• Samples from Penn Treebank.

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• Chomsky Normal Form

• A CFG is in Chomsky Normal Form (CNF) if each production is either of the form A → B C or A → a. That is, the right-hand side of each rule either has two non-terminal symbols or one terminal symbol.

• Conversion to CNF

VP → VBD NP PPVP → VP PPVP -> VBD NP PP*

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依存语法Dependency Grammar

• Definition

• It is a kind of grammar where the syntactic structure of a sentence is described purely in terms of words and binary semantic or syntactic relations between these words.

• Dependency relations are directional.

• There are no structural levels or non-terminal nodes as in CFG.

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• A Chinese Example

Dependency Tree

• Dependency Graph

那个小孩喜欢通俗歌曲

喜欢

小孩 歌曲

通俗那个

喜欢小孩 歌曲通俗Root

HED

SBV

VOB

ATT

那个

ATT

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• Axioms of Dependency

• Only one constituent in a sentence is independent.

• All the other constituents in the sentence are dependent on some constituent.

• No constituent is dependent on two or more other constituents.

• If A is dependent on B and C is situated between A and B in the sentence, then either C is dependent on A or B, or C is dependent on a constituent between A and B.

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• Conditions of Dependency Tree

• Single Type Node: A dependency tree has only terminal nodes and no non-terminal nodes.

• Single Parent Node: The root node is the only parent node. All the other nodes have only one parent node.

• Unique Root Node: A dependency tree has only one root node, which governs all the other nodes.

• Non-overlapping: A dependency tree’s branches cannot overlap with each other.

• Mutual exclusiveness: The relations of governing and preceding are exclusive. If two nodes have a “governing” relation between them, they cannot have a “preceding” relation.

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• Dependency Relations

• There are more than 50 dependency relations in English (Stanford Parser)

Dependency relation

Meaning Example

amod adjectival modifier Sam eats red meatamod(meat, red)

dobj direct object She gave me a raisedobj(gave, raise)

nsubj nominal subject Clinton defeated Dole nsubj (defeated, Clinton)

pcomp prepositional complement

They heard about you missing classes pcomp(about, missing)

tmod temporal modifier Last night, I swam in the pooltmod(swam, night)

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In-Class Exercise

• Given the sentence The sausage was eaten by his dog, complete the following dependency relations by choosing from the list of {nsubj, amod, dobj, pcomp, tmod}.

_____(eat, sausage)

_____(eat, dog)

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• Heads and Dependency

• Syntactic constituents could be associated with a lexical head.

• N is the head of an NP, V is the head of a VP …

Workers dumped sacks into a bin.

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• Heads and Dependency

• A dependency graph can be automatically derived from a context-free parse by using the head rules.

Vinken will join the board as a nonexecutive director Nov 29.

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• 语法分析基础• Constituents

• 形式语法• Regular Expressions

• Symbols and Rules

• Formal Definition

• 上下文无关语法• Parse Tree

Wrap-Up

• Examples

• Treebanks

• 依存语法• Axioms

• Dependency Tree and Graph

• Dependency Relations

• Heads and Dependency