【導讀】詞、句子或更大的一段文字組合的“自然”語言，如英語。器語言，前者通常被稱為“自然語言”。在機器語言中，理解輸入的自然語言通常指。恰當?shù)妮斎雱幼鳌Ｗ匀徽Z言能說出和寫的一樣的意思。那是因為演講聽起來是和分割印刷在紙上的。是不一樣的，理解話語有著額外的困難，我會在一個后來的篇章來描述。自然語言理解的逆輸入生成自然語言輸出的口語和書寫。進行一場談話也是如此。所有的這些問題-理解、產(chǎn)生、翻譯與一般的談話——構成了“自然語言處理”。語言學家和其他學習語言者對語言的認識和分析處在不同級別上的。交易形態(tài)與全字是如何加在一起，從小型零件的制造和銷售。接下來，語法關注句子的結構和語法。它規(guī)定所試圖描述一串字在一個特定的語。例如，該字符串“約翰打了球”是語法，但是“球到約翰”。能被認為語法，但它是荒謬的。則，可以設置可用于生成“合法”的一種語言。這些句法取代通過額外的象征符號形成語法的規(guī)律琴弦。

　　

【正文】 sed by puters， the former are usually called“natural languages.”In artificial intelligence， “understanding”natural language input usually means either converting it to some kind of memory model(such as the one used by Raphael in his SIR system or the semantic work used by Quillian)or the evocation of some action appropriate to the input. Natural languages are spoken as well as ， because speech sounds are not as well segmented as are the characters printed on a page， speech understanding presents additional di?culties， which I?ll describe in a later chapter. The inverse of understanding natural language input is generating natural language output–both written and from one language to another involves both understanding and does carrying on a of these problems understanding， generation， translation， and conversing–fall under the general heading of“natural language processing”(sometimes abbreviated as NLP). Linguistic Levels Linguists and others who study language recognize several levels at which language can be levels can be arranged in a sort of hierarchy， starting with those dealing with the most basic ponents of language (sounds and word parts)and proceeding upward to levels dealing with sequences of speech is being dealt with， there are the levels of phoics(language sounds) and phonology (organization of sounds into words).For both speech and text， morphology deals with how whole words are put together from smaller example， “walking” consists of “walk” plus “ing.” Next， syntax is concerned with sentence structure and attempts to describe rules by which a string of words in a certain language can be labeled either grammatical or example， the string“John hit the ball”is grammatical but the string“ball the hit John”is with the dictionary definitions of words， syntax es next in importance for understanding the meaning of a example， the sentence“John saw the man with a telescope”has two di?erent meanings depending on its syntactic structure(that is， depending on whether“with a telescope”refers to“ the man”who had a telescope or to“saw.”) But grammaticality alone is insu?cient for determining example ， the sentence“Colorless green ideas sleep furiously”might be considered grammatical， but it is semantics level helps to determine the meaning(or the meaninglessness)of a sentence by employing logical example， through semantic analysis， an“idea”can?t be both“colorless”and“green.” Next es the pragmatics level， which considers the context of a sentence to pin down example， “John went to the bank”would have a di?erent meaning in a sentence about stream fishing than it would in a sentence about deals with meanings in the context of specific situations. One of these levels in particular， namely， syntax， was the subject of much early study and continues to be an important aspect of 1957， the American linguist Noam Chomsky published a groundbreaking book titled Syntactic Structures in which he proposed sets of grammatical rules that could be used for generating the“l(fā)egal”sentences of a The same rules could also be used to analyze a string of words to determine whether or not they formed a legal sentence of the ?ll illustrate how this analysis is done using what Chomsky called a phrasestructure grammar(PSG).2 The process is very similar to how we all“diagrammed”sentences back in grade school. Grammars are defined by stating rules for replacing words in the string by symbols corresponding to syntactic categories， such as noun or verb or also have rules for replacing strings of these syntactic symbols by additional illustrate these ideas， I?ll use a very simple grammar adapted from one of Chomsky?s grammar has only three syntactic categories:determiner ， noun， and three are su?cient for analysing strings such as“the man hit the ball.” One of the rules in this illustrative grammar states that we can replace either of the words“the”or“a”by the symbol“DET”(for determiner). Linguists write this rule as follows: the|a→ DET (The symbol|is used to indicate that either of the words that surround it can be replaced by the syntactic symbol to the right of the arrow.) Here are some other rules， written in the same format: man|ball|john→ N (The words“man， ”“ball， ”and“john”can be replaced by the symbol“ N”for noun.) hit|took|threw→ V (The words“hit， ”“took， ”and“threw”can be replaced by the symbol“ V”for verb.) DET N→ NP (The string of symbols“DET”and“N”can be replaced by the symbol“ NP”for noun phrase.) V NP→ VP (The string of symbols“V”and“NP”can be replaced by the symbol“ VP”for verb phrase.) NP VP→ S (The string of symbols“NP”and“VP”can be replaced by the symbol“ S”for sentence.) Symbols such as“S， ”“DET， ”“NP， ”and so on are called the“nonterminal” symbols of the language defined by the grammar， whereas vocabulary words such as “ball， ” “john， ”and“threw”are the“terminal”symbols of the language. We can apply these rules to the string“the man hit the ball”to transform it into“S.”Any string that can be changed into“S”in this way is said to be grammatical–a legal sentence in the language defined by this very simple way to illustrate the rule applications， called a parse tree， is shown in Fig. This example was based on a small set of syntactic categories and replacement rules just to illustrate the main ideas about syntactic make the grammar slightly more realistic， we would need to include symbols and replacement rules for adjectives， adverbs，prepositions， and so ， of course， we would have to include many more vocabulary words. Grammars are called contextfree grammars(CFGs)if all of their rules have just a single nonterminal symbol on the right side of the are called that because when the rules are used in reverse(to generate rather than to analyze grammatical sentences)， the way in which a nonterminal symbol is replaced