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Grammar induction, also known as grammatical inference or syntactic pattern recognition, refers to the process in machine learning of learning a formal grammar (usually as a collection of re-write rules or productions or alternatively as a finite state machine or automaton of some kind) from a set of observations, thus constructing a model which accounts for the characteristics of the observed objects. More generally, grammatical inference is that branch of machine learning where the instance space consists of discrete combinatorial objects such as strings, trees and graphs.
There is now a rich literature on learning different types of grammar and automata, under various different learning models and using various different methodologies.
Grammatical inference has often been very focused on the problem of learning finite state machines of various types (see the article Induction of regular languages for details on these approaches), since there have been efficient algorithms for this problem since the 1980s.
More recently these approaches have been extended to the problem of inference of context-free grammars and richer formalisms, such as multiple context-free grammars and parallel multiple context-free grammars. Other classes of grammars for which grammatical inference has been studied are contextual grammars, and pattern languages.
The simplest form of learning is where the learning algorithm merely receives a set of examples drawn from the language in question, but other learning models have been studied. One frequently studied alternative is the case where the learner can ask membership queries as in the exact query learning model or minimally adequate teacher model introduced by Angluin.
There are a wide variety of methods for grammatical inference. Two of the classic sources are Fu (1977) and Fu (1982). Duda, Hart & Stork (2001) also devote a brief section to the problem, and cite a number of references. The basic trial-and-error method they present is discussed below. For approaches to infer subclasses of regular languages in particular, see Induction of regular languages. A more recent textbook is de la Higuera (2010)  which covers the theory of grammatical inference of regular languages and finite state automata.
Grammatical inference by trial-and-error
The method proposed in Section 8.7 of Duda, Hart & Stork (2001) suggests successively guessing grammar rules (productions) and testing them against positive and negative observations. The rule set is expanded so as to be able to generate each positive example, but if a given rule set also generates a negative example, it must be discarded. This particular approach can be characterized as "hypothesis testing" and bears some similarity to Mitchel's version space algorithm. The Duda, Hart & Stork (2001) text provide a simple example which nicely illustrates the process, but the feasibility of such an unguided trial-and-error approach for more substantial problems is dubious.
Grammatical inference by genetic algorithms
Grammatical Induction using evolutionary algorithms is the process of evolving a representation of the grammar of a target language through some evolutionary process. Formal grammars can easily be represented as a tree structure of production rules that can be subjected to evolutionary operators. Algorithms of this sort stem from the genetic programming paradigm pioneered by John Koza. Other early work on simple formal languages used the binary string representation of genetic algorithms, but the inherently hierarchical structure of grammars couched in the EBNF language made trees a more flexible approach.
Koza represented Lisp programs as trees. He was able to find analogues to the genetic operators within the standard set of tree operators. For example, swapping sub-trees is equivalent to the corresponding process of genetic crossover, where sub-strings of a genetic code are transplanted into an individual of the next generation. Fitness is measured by scoring the output from the functions of the lisp code. Similar analogues between the tree structured lisp representation and the representation of grammars as trees, made the application of genetic programming techniques possible for grammar induction.
In the case of Grammar Induction, the transplantation of sub-trees corresponds to the swapping of production rules that enable the parsing of phrases from some language. The fitness operator for the grammar is based upon some measure of how well it performed in parsing some group of sentences from the target language. In a tree representation of a grammar, a terminal symbol of a production rule corresponds to a leaf node of the tree. Its parent nodes corresponds to a non-terminal symbol (e.g. a noun phrase or a verb phrase) in the rule set. Ultimately, the root node might correspond to a sentence non-terminal.
Grammatical inference by greedy algorithms
Like all greedy algorithms, greedy grammar inference algorithms make, in iterative manner, decisions that seem to be the best at that stage. These made decisions deal usually with things like the making of a new or the removing of the existing rules, the choosing of the applied rule or the merging of some existing rules. Because there are several ways to define 'the stage' and 'the best', there are also several greedy grammar inference algorithms.
These context-free grammar generating algorithms make the decision after every read symbol:
- Lempel-Ziv-Welch algorithm creates a context-free grammar in a deterministic way such that it is necessary to store only the start rule of the generated grammar.
- Sequitur and its modifications.
These context-free grammar generating algorithms first read the whole given symbol-sequence and then start to make decisions:
- Byte pair encoding and its optimizations.
A more recent approach is based on Distributional Learning. Algorithms using these approaches have been applied to learning context-free grammars and mildly context-sensitive languages and have been proven to be correct and efficient for large subclasses of these grammars.
The principle of grammar induction has been applied to other aspects of natural language processing, and have been applied (among many other problems) to morpheme analysis, and even place name derivations. Grammar induction has also been used for lossless data compression and statistical inference via MML and MDL principles.
- Artificial grammar learning
- Syntactic pattern recognition
- Inductive inference
- Straight-line grammar
- Kolmogorov complexity
- Automatic distillation of structure
- Inductive programming
- de la Higuera, Colin (2010). Grammatical Inference: Learning Automata and Grammars. Cambridge: Cambridge University Press.
- Clark and Eyraud (2007) Journal of Machine Learning Research, Ryo Yoshinaka (2011) Theoretical Computer Science
- Duda, Richard O.; Hart, Peter E.; Stork, David G. (2001), Pattern Classification (2 ed.), New York: John Wiley & Sons
- Fu, King Sun (1982), Syntactic Pattern Recognition and Applications, Englewood Cliffs, NJ: Prentice-Hall
- Fu, King Sun (1977), Syntactic Pattern Recognition, Applications, Berlin: Springer-Verlag
- Horning, James Jay (1969), A Study of Grammatical Inference (Ph.D. Thesis ed.), Stanford: Stanford University Computer Science Department
- Gold, E. Mark (1967), Language Identification in the Limit 10, Information and Control, pp. 447–474, see also the corresponding Wikipedia article