Natural language processing has its roots in the 1950s.[2] Already in 1950,Alan Turing published an article titled "Computing Machinery and Intelligence" which proposed what is now called theTuring test as a criterion of intelligence, though at the time that was not articulated as a problem separate from artificial intelligence. The proposed test includes a task that involves the automated interpretation and generation of natural language.
The premise of symbolic NLP is well-summarized byJohn Searle'sChinese room experiment: Given a collection of rules (e.g., a Chinese phrasebook, with questions and matching answers), the computer emulates natural language understanding (or other NLP tasks) by applying those rules to the data it confronts.
1950s: TheGeorgetown experiment in 1954 involved fullyautomatic translation of more than sixty Russian sentences into English. The authors claimed that within three or five years, machine translation would be a solved problem.[3] However, real progress was much slower, and after theALPAC report in 1966, which found that ten years of research had failed to fulfill the expectations, funding for machine translation was dramatically reduced. Little further research in machine translation was conducted in America (though some research continued elsewhere, such as Japan and Europe[4]) until the late 1980s when the firststatistical machine translation systems were developed.
1960s: Some notably successful natural language processing systems developed in the 1960s wereSHRDLU, a natural language system working in restricted "blocks worlds" with restricted vocabularies, andELIZA, a simulation of aRogerian psychotherapist, written byJoseph Weizenbaum between 1964 and 1966. Using almost no information about human thought or emotion, ELIZA sometimes provided a startlingly human-like interaction. When the "patient" exceeded the very small knowledge base, ELIZA might provide a generic response, for example, responding to "My head hurts" with "Why do you say your head hurts?". Ross Quillian's successful work on natural language was demonstrated with a vocabulary of onlytwenty words, because that was all that would fit in a computer memory at the time.[5]
1970s: During the 1970s, many programmers began to write "conceptualontologies", which structured real-world information into computer-understandable data. Examples are MARGIE (Schank, 1975), SAM (Cullingford, 1978), PAM (Wilensky, 1978), TaleSpin (Meehan, 1976), QUALM (Lehnert, 1977), Politics (Carbonell, 1979), and Plot Units (Lehnert 1981). During this time, the firstchatterbots were written (e.g.,PARRY).
1980s: The 1980s and early 1990s mark the heyday of symbolic methods in NLP. Focus areas of the time included research on rule-based parsing (e.g., the development ofHPSG as a computational operationalization ofgenerative grammar), morphology (e.g., two-level morphology[6]), semantics (e.g.,Lesk algorithm), reference (e.g., within Centering Theory[7]) and other areas of natural language understanding (e.g., in theRhetorical Structure Theory). Other lines of research were continued, e.g., the development of chatterbots withRacter andJabberwacky. An important development (that eventually led to the statistical turn in the 1990s) was the rising importance of quantitative evaluation in this period.[8]
Up until the 1980s, most natural language processing systems were based on complex sets of hand-written rules. Starting in the late 1980s, however, there was a revolution in natural language processing with the introduction ofmachine learning algorithms for language processing. This was due to both the steady increase in computational power (seeMoore's law) and the gradual lessening of the dominance ofChomskyan theories of linguistics (e.g.transformational grammar), whose theoretical underpinnings discouraged the sort ofcorpus linguistics that underlies the machine-learning approach to language processing.[9]
1990s: Many of the notable early successes in statistical methods in NLP occurred in the field ofmachine translation, due especially to work at IBM Research, such asIBM alignment models. These systems were able to take advantage of existing multilingualtextual corpora that had been produced by theParliament of Canada and theEuropean Union as a result of laws calling for the translation of all governmental proceedings into all official languages of the corresponding systems of government. However, most other systems depended on corpora specifically developed for the tasks implemented by these systems, which was (and often continues to be) a major limitation in the success of these systems. As a result, a great deal of research has gone into methods of more effectively learning from limited amounts of data.
2000s: With the growth of the web, increasing amounts of raw (unannotated) language data have become available since the mid-1990s. Research has thus increasingly focused onunsupervised andsemi-supervised learning algorithms. Such algorithms can learn from data that has not been hand-annotated with the desired answers or using a combination of annotated and non-annotated data. Generally, this task is much more difficult thansupervised learning, and typically produces less accurate results for a given amount of input data. However, there is an enormous amount of non-annotated data available (including, among other things, the entire content of theWorld Wide Web), which can often make up for the worse efficiency if the algorithm used has a low enoughtime complexity to be practical.
2003:word n-gram model, at the time the best statistical algorithm, is outperformed by amulti-layer perceptron (with a single hidden layer and context length of several words, trained on up to 14 million words, byBengio et al.)[10]
Symbolic approach, i.e., the hand-coding of a set of rules for manipulating symbols, coupled with a dictionary lookup, was historically the first approach used both by AI in general and by NLP in particular:[19][20] such as by writing grammars or devising heuristic rules forstemming.
Machine learning approaches, which include both statistical and neural networks, on the other hand, have many advantages over the symbolic approach:
both statistical and neural networks methods can focus more on the most common cases extracted from a corpus of texts, whereas the rule-based approach needs to provide rules for both rare cases and common ones equally.
language models, produced by either statistical or neural networks methods, are more robust to both unfamiliar (e.g. containing words or structures that have not been seen before) and erroneous input (e.g. with misspelled words or words accidentally omitted) in comparison to the rule-based systems, which are also more costly to produce.
the larger such a (probabilistic) language model is, the more accurate it becomes, in contrast to rule-based systems that can gain accuracy only by increasing the amount and complexity of the rules leading tointractability problems.
Rule-based systems are commonly used:
when the amount of training data is insufficient to successfully apply machine learning methods, e.g., for the machine translation of low-resource languages such as provided by theApertium system,
for preprocessing in NLP pipelines, e.g.,tokenization, or
for postprocessing and transforming the output of NLP pipelines, e.g., forknowledge extraction from syntactic parses.
In the late 1980s and mid-1990s, the statistical approach ended a period ofAI winter, which was caused by the inefficiencies of the rule-based approaches.[21][22]
The earliestdecision trees, producing systems of hardif–then rules, were still very similar to the old rule-based approaches.Only the introduction of hiddenMarkov models, applied to part-of-speech tagging, announced the end of the old rule-based approach.
The following is a list of some of the most commonly researched tasks in natural language processing. Some of these tasks have direct real-world applications, while others more commonly serve as subtasks that are used to aid in solving larger tasks.
Though natural language processing tasks are closely intertwined, they can be subdivided into categories for convenience. A coarse division is given below.
Given a sound clip of a person or people speaking, determine the textual representation of the speech. This is the opposite oftext to speech and is one of the extremely difficult problems colloquially termed "AI-complete" (see above). Innatural speech there are hardly any pauses between successive words, and thusspeech segmentation is a necessary subtask of speech recognition (see below). In most spoken languages, the sounds representing successive letters blend into each other in a process termedcoarticulation, so the conversion of theanalog signal to discrete characters can be a very difficult process. Also, given that words in the same language are spoken by people with different accents, the speech recognition software must be able to recognize the wide variety of input as being identical to each other in terms of its textual equivalent.
Tokenization is a process used in text analysis that divides text into individual words or word fragments. This technique results in two key components: a word index and tokenized text. The word index is a list that maps unique words to specific numerical identifiers, and the tokenized text replaces each word with its corresponding numerical token. These numerical tokens are then used in various deep learning methods.[26]
For a language likeEnglish, this is fairly trivial, since words are usually separated by spaces. However, some written languages likeChinese,Japanese andThai do not mark word boundaries in such a fashion, and in those languages text segmentation is a significant task requiring knowledge of thevocabulary andmorphology of words in the language. Sometimes this process is also used in cases likebag of words (BOW) creation in data mining.[citation needed]
The task of removing inflectional endings only and to return the base dictionary form of a word which is also known as a lemma. Lemmatization is another technique for reducing words to their normalized form. But in this case, the transformation actually uses a dictionary to map words to their actual form.[27]
Separate words into individualmorphemes and identify the class of the morphemes. The difficulty of this task depends greatly on the complexity of themorphology (i.e., the structure of words) of the language being considered.English has fairly simple morphology, especiallyinflectional morphology, and thus it is often possible to ignore this task entirely and simply model all possible forms of a word (e.g., "open, opens, opened, opening") as separate words. In languages such asTurkish orMeitei, a highlyagglutinated Indian language, however, such an approach is not possible, as each dictionary entry has thousands of possible word forms.[28]
Given a sentence, determine thepart of speech (POS) for each word. Many words, especially common ones, can serve as multiple parts of speech. For example, "book" can be anoun ("the book on the table") orverb ("to book a flight"); "set" can be a noun, verb oradjective; and "out" can be any of at least five different parts of speech.
The process of reducing inflected (or sometimes derived) words to a base form (e.g., "close" will be the root for "closed", "closing", "close", "closer" etc.). Stemming yields similar results as lemmatization, but does so on grounds of rules, not a dictionary.
Given a chunk of text, find the sentence boundaries. Sentence boundaries are often marked byperiods or otherpunctuation marks, but these same characters can serve other purposes (e.g., markingabbreviations).
Determine theparse tree (grammatical analysis) of a given sentence. Thegrammar fornatural languages isambiguous and typical sentences have multiple possible analyses: perhaps surprisingly, for a typical sentence there may be thousands of potential parses (most of which will seem completely nonsensical to a human). There are two primary types of parsing:dependency parsing andconstituency parsing. Dependency parsing focuses on the relationships between words in a sentence (marking things like primary objects and predicates), whereas constituency parsing focuses on building out the parse tree using aprobabilistic context-free grammar (PCFG) (see alsostochastic grammar).
Lexical semantics (of individual words in context)
Given a stream of text, determine which items in the text map to proper names, such as people or places, and what the type of each such name is (e.g. person, location, organization). Althoughcapitalization can aid in recognizing named entities in languages such as English, this information cannot aid in determining the type ofnamed entity, and in any case, is often inaccurate or insufficient. For example, the first letter of a sentence is also capitalized, and named entities often span several words, only some of which are capitalized. Furthermore, many other languages in non-Western scripts (e.g.Chinese orArabic) do not have any capitalization at all, and even languages with capitalization may not consistently use it to distinguish names. For example,German capitalizes allnouns, regardless of whether they are names, andFrench andSpanish do not capitalize names that serve asadjectives. Another name for this task is token classification.[30]
Sentiment analysis is a computational method used to identify and classify the emotional intent behind text. This technique involves analyzing text to determine whether the expressed sentiment is positive, negative, or neutral. Models for sentiment classification typically utilize inputs such asword n-grams,Term Frequency-Inverse Document Frequency (TF-IDF) features, hand-generated features, or employdeep learning models designed to recognize both long-term and short-term dependencies in text sequences. The applications of sentiment analysis are diverse, extending to tasks such as categorizing customer reviews on various online platforms.[26]
Many words have more than onemeaning; we have to select the meaning which makes the most sense in context. For this problem, we are typically given a list of words and associated word senses, e.g. from a dictionary or an online resource such asWordNet.
Many words—typically proper names—refer tonamed entities; here we have to select the entity (a famous individual, a location, a company, etc.) which is referred to in context.
Relational semantics (semantics of individual sentences)
Given a piece of text (typically a sentence), produce a formal representation of its semantics, either as a graph (e.g., inAMR parsing) or in accordance with a logical formalism (e.g., inDRT parsing). This challenge typically includes aspects of several more elementary NLP tasks from semantics (e.g., semantic role labelling, word-sense disambiguation) and can be extended to include full-fledged discourse analysis (e.g., discourse analysis, coreference; seeNatural language understanding below).
Given a single sentence, identify and disambiguate semantic predicates (e.g., verbalframes), then identify and classify the frame elements (semantic roles).
Given a sentence or larger chunk of text, determine which words ("mentions") refer to the same objects ("entities").Anaphora resolution is a specific example of this task, and is specifically concerned with matching uppronouns with the nouns or names to which they refer. The more general task of coreference resolution also includes identifying so-called "bridging relationships" involvingreferring expressions. For example, in a sentence such as "He entered John's house through the front door", "the front door" is a referring expression and the bridging relationship to be identified is the fact that the door being referred to is the front door of John's house (rather than of some other structure that might also be referred to).
This rubric includes several related tasks. One task is discourse parsing, i.e., identifying thediscourse structure of a connected text, i.e. the nature of the discourse relationships between sentences (e.g. elaboration, explanation, contrast). Another possible task is recognizing and classifying thespeech acts in a chunk of text (e.g. yes–no question, content question, statement, assertion, etc.).
Implicit semantic role labelling
Given a single sentence, identify and disambiguate semantic predicates (e.g., verbalframes) and their explicit semantic roles in the current sentence (seeSemantic role labelling above). Then, identify semantic roles that are not explicitly realized in the current sentence, classify them into arguments that are explicitly realized elsewhere in the text and those that are not specified, and resolve the former against the local text. A closely related task is zero anaphora resolution, i.e., the extension of coreference resolution topro-drop languages.
Given two text fragments, determine if one being true entails the other, entails the other's negation, or allows the other to be either true or false.[31]
The goal of argument mining is the automatic extraction and identification of argumentative structures fromnatural language text with the aid of computer programs.[32] Such argumentative structures include the premise, conclusions, theargument scheme and the relationship between the main and subsidiary argument, or the main and counter-argument within discourse.[33][34]
Produce a readable summary of a chunk of text. Often used to provide summaries of the text of a known type, such as research papers, articles in the financial section of a newspaper.
Grammatical error correction
Grammatical error detection and correction involves a great band-width of problems on all levels of linguistic analysis (phonology/orthography, morphology, syntax, semantics, pragmatics). Grammatical error correction is impactful since it affects hundreds of millions of people that use or acquire English as a second language. It has thus been subject to a number of shared tasks since 2011.[35][36][37] As far as orthography, morphology, syntax and certain aspects of semantics are concerned, and due to the development of powerful neural language models such asGPT-2, this can now (2019) be considered a largely solved problem and is being marketed in various commercial applications.
Automatically translate text from one human language to another. This is one of the most difficult problems, and is a member of a class of problems colloquially termed "AI-complete", i.e. requiring all of the different types of knowledge that humans possess (grammar, semantics, facts about the real world, etc.) to solve properly.
Convert chunks of text into more formal representations such asfirst-order logic structures that are easier forcomputer programs to manipulate. Natural language understanding involves the identification of the intended semantic from the multiple possible semantics which can be derived from a natural language expression which usually takes the form of organized notations of natural language concepts. Introduction and creation of language metamodel and ontology are efficient however empirical solutions. An explicit formalization of natural language semantics without confusions with implicit assumptions such asclosed-world assumption (CWA) vs.open-world assumption, or subjective Yes/No vs. objective True/False is expected for the construction of a basis of semantics formalization.[38]
Convert information from computer databases or semantic intents into readable human language.
Book generation
Not an NLP task proper but an extension of natural language generation and other NLP tasks is the creation of full-fledged books. The first machine-generated book was created by a rule-based system in 1984 (Racter,The policeman's beard is half-constructed).[39] The first published work by a neural network was published in 2018,1 the Road, marketed as a novel, contains sixty million words. Both these systems are basically elaborate but non-sensical (semantics-free)language models. The first machine-generated science book was published in 2019 (Beta Writer,Lithium-Ion Batteries, Springer, Cham).[40] UnlikeRacter and1 the Road, this is grounded on factual knowledge and based on text summarization.
A Document AI platform sits on top of the NLP technology enabling users with no prior experience of artificial intelligence, machine learning or NLP to quickly train a computer to extract the specific data they need from different document types. NLP-powered Document AI enables non-technical teams to quickly access information hidden in documents, for example, lawyers, business analysts and accountants.[41]
Given a human-language question, determine its answer. Typical questions have a specific right answer (such as "What is the capital of Canada?"), but sometimes open-ended questions are also considered (such as "What is the meaning of life?").
Based on long-standing trends in the field, it is possible to extrapolate future directions of NLP. As of 2020, three trends among the topics of the long-standing series of CoNLL Shared Tasks can be observed:[47]
Interest on increasingly abstract, "cognitive" aspects of natural language (1999–2001: shallow parsing, 2002–03: named entity recognition, 2006–09/2017–18: dependency syntax, 2004–05/2008–09 semantic role labelling, 2011–12 coreference, 2015–16: discourse parsing, 2019: semantic parsing).
Increasing interest in multilinguality, and, potentially, multimodality (English since 1999; Spanish, Dutch since 2002; German since 2003; Bulgarian, Danish, Japanese, Portuguese, Slovenian, Swedish, Turkish since 2006; Basque, Catalan, Chinese, Greek, Hungarian, Italian, Turkish since 2007; Czech since 2009; Arabic since 2012; 2017: 40+ languages; 2018: 60+/100+ languages)
Elimination of symbolic representations (rule-based over supervised towards weakly supervised methods, representation learning and end-to-end systems)
Most higher-level NLP applications involve aspects that emulate intelligent behaviour and apparent comprehension of natural language. More broadly speaking, the technical operationalization of increasingly advanced aspects of cognitive behaviour represents one of the developmental trajectories of NLP (see trends among CoNLL shared tasks above).
Cognition refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses."[48]Cognitive science is the interdisciplinary, scientific study of the mind and its processes.[49]Cognitive linguistics is an interdisciplinary branch of linguistics, combining knowledge and research from both psychology and linguistics.[50] Especially during the age ofsymbolic NLP, the area of computational linguistics maintained strong ties with cognitive studies.
As an example,George Lakoff offers a methodology to build natural language processing (NLP) algorithms through the perspective of cognitive science, along with the findings of cognitive linguistics,[51] with two defining aspects:
Apply the theory ofconceptual metaphor, explained by Lakoff as "the understanding of one idea, in terms of another" which provides an idea of the intent of the author.[52] For example, consider the English wordbig. When used in a comparison ("That is a big tree"), the author's intent is to imply that the tree isphysically large relative to other trees or the authors experience. When used metaphorically ("Tomorrow is a big day"), the author's intent to implyimportance. The intent behind other usages, like in "She is a big person", will remain somewhat ambiguous to a person and a cognitive NLP algorithm alike without additional information.
Assign relative measures of meaning to a word, phrase, sentence or piece of text based on the information presented before and after the piece of text being analyzed, e.g., by means of aprobabilistic context-free grammar (PCFG). The mathematical equation for such algorithms is presented inUS Patent 9269353:[53]
Where
RMM is the relative measure of meaning
token is any block of text, sentence, phrase or word
N is the number of tokens being analyzed
PMM is the probable measure of meaning based on a corpora
d is the non zero location of the token along the sequence ofN tokens
PF is the probability function specific to a language
Ties with cognitive linguistics are part of the historical heritage of NLP, but they have been less frequently addressed since the statistical turn during the 1990s. Nevertheless, approaches to develop cognitive models towards technically operationalizable frameworks have been pursued in the context of various frameworks, e.g., of cognitive grammar,[54] functional grammar,[55] construction grammar,[56] computational psycholinguistics and cognitive neuroscience (e.g.,ACT-R), however, with limited uptake in mainstream NLP (as measured by presence on major conferences[57] of theACL). More recently, ideas of cognitive NLP have been revived as an approach to achieveexplainability, e.g., under the notion of "cognitive AI".[58] Likewise, ideas of cognitive NLP are inherent to neural modelsmultimodal NLP (although rarely made explicit)[59] and developments inartificial intelligence, specifically tools and technologies usinglarge language model approaches[60] and new directions inartificial general intelligence based on thefree energy principle[61] by British neuroscientist and theoretician at University College LondonKarl J. Friston.
^"ALPAC: the (in)famous report", John Hutchins, MT News International, no. 14, June 1996, pp. 9–12.
^Crevier 1993, pp. 146–148 harvnb error: no target: CITEREFCrevier1993 (help), see alsoBuchanan 2005, p. 56 harvnb error: no target: CITEREFBuchanan2005 (help): "Early programs were necessarily limited in scope by the size and speed of memory"
^Chomskyan linguistics encourages the investigation of "corner cases" that stress the limits of its theoretical models (comparable topathological phenomena in mathematics), typically created usingthought experiments, rather than the systematic investigation of typical phenomena that occur in real-world data, as is the case incorpus linguistics. The creation and use of suchcorpora of real-world data is a fundamental part of machine-learning algorithms for natural language processing. In addition, theoretical underpinnings of Chomskyan linguistics such as the so-called "poverty of the stimulus" argument entail that general learning algorithms, as are typically used in machine learning, cannot be successful in language processing. As a result, the Chomskyan paradigm discouraged the application of such models to language processing.
^Bengio, Yoshua; Ducharme, Réjean; Vincent, Pascal; Janvin, Christian (March 1, 2003)."A neural probabilistic language model".The Journal of Machine Learning Research.3:1137–1155 – via ACM Digital Library.
^Goldberg, Yoav (2016). "A Primer on Neural Network Models for Natural Language Processing".Journal of Artificial Intelligence Research.57:345–420.arXiv:1807.10854.doi:10.1613/jair.4992.S2CID8273530.
^Goodfellow, Ian; Bengio, Yoshua; Courville, Aaron (2016).Deep Learning. MIT Press.
^Jozefowicz, Rafal; Vinyals, Oriol; Schuster, Mike; Shazeer, Noam; Wu, Yonghui (2016).Exploring the Limits of Language Modeling.arXiv:1602.02410.Bibcode:2016arXiv160202410J.
^Schank, Roger C.; Abelson, Robert P. (1977).Scripts, Plans, Goals, and Understanding: An Inquiry Into Human Knowledge Structures. Hillsdale: Erlbaum.ISBN0-470-99033-3.
^Socher, Richard."Deep Learning For NLP-ACL 2012 Tutorial".www.socher.org. Archived fromthe original on 2021-04-14. Retrieved2020-08-17. This was an early Deep Learning tutorial at the ACL 2012 and met with both interest and (at the time) skepticism by most participants. Until then, neural learning was basically rejected because of its lack of statistical interpretability. Until 2015, deep learning had evolved into the major framework of NLP. [Link is broken, tryhttp://web.stanford.edu/class/cs224n/]
^Kishorjit, N.; Vidya, Raj RK.; Nirmal, Y.; Sivaji, B. (2012)."Manipuri Morpheme Identification"(PDF).Proceedings of the 3rd Workshop on South and Southeast Asian Natural Language Processing (SANLP). COLING 2012, Mumbai, December 2012:95–108.{{cite journal}}: CS1 maint: location (link)
^Coyne, Bob; Sproat, Richard (2001-08-01)."WordsEye".Proceedings of the 28th annual conference on Computer graphics and interactive techniques. SIGGRAPH '01. New York, NY, USA: Association for Computing Machinery. pp. 487–496.doi:10.1145/383259.383316.ISBN978-1-58113-374-5.S2CID3842372.
^"Ask the Cognitive Scientist".American Federation of Teachers. 8 August 2014.Cognitive science is an interdisciplinary field of researchers from Linguistics, psychology, neuroscience, philosophy, computer science, and anthropology that seek to understand the mind.
^Robinson, Peter (2008).Handbook of Cognitive Linguistics and Second Language Acquisition. Routledge. pp. 3–8.ISBN978-0-805-85352-0.
^Lakoff, George (1999).Philosophy in the Flesh: The Embodied Mind and Its Challenge to Western Philosophy; Appendix: The Neural Theory of Language Paradigm. New York Basic Books. pp. 569–583.ISBN978-0-465-05674-3.
^Strauss, Claudia (1999).A Cognitive Theory of Cultural Meaning. Cambridge University Press. pp. 156–164.ISBN978-0-521-59541-4.
^Dasgupta, Ishita; Lampinen, Andrew K.; Chan, Stephanie C. Y.; Creswell, Antonia; Kumaran, Dharshan; McClelland, James L.; Hill, Felix (2022). "Language models show human-like content effects on reasoning, Dasgupta, Lampinen et al".arXiv:2207.07051 [cs.CL].
^Friston, Karl J. (2022).Active Inference: The Free Energy Principle in Mind, Brain, and Behavior; Chapter 4 The Generative Models of Active Inference. The MIT Press.ISBN978-0-262-36997-8.
Steven Bird, Ewan Klein, and Edward Loper (2009).Natural Language Processing with Python. O'Reilly Media.ISBN978-0-596-51649-9.
Kenna Hughes-Castleberry, "A Murder Mystery Puzzle: The literary puzzleCain's Jawbone, which has stumped humans for decades, reveals the limitations of natural-language-processing algorithms",Scientific American, vol. 329, no. 4 (November 2023), pp. 81–82. "This murder mystery competition has revealed that although NLP (natural-language processing) models are capable of incredible feats, their abilities are very much limited by the amount ofcontext they receive. This [...] could cause [difficulties] for researchers who hope to use them to do things such as analyzeancient languages. In some cases, there are few historical records on long-gonecivilizations to serve astraining data for such a purpose." (p. 82.)
Daniel Jurafsky and James H. Martin (2008).Speech and Language Processing, 2nd edition. Pearson Prentice Hall.ISBN978-0-13-187321-6.
Mohamed Zakaria Kurdi (2016).Natural Language Processing and Computational Linguistics: speech, morphology, and syntax, Volume 1. ISTE-Wiley.ISBN978-1848218482.
Mohamed Zakaria Kurdi (2017).Natural Language Processing and Computational Linguistics: semantics, discourse, and applications, Volume 2. ISTE-Wiley.ISBN978-1848219212.
