Within the fields of computer science and linguistics, specifically in the area of formal languages, the Chomsky hierarchy (occasionally referred to as ChomskySchützenberger hierarchy) is a containment hierarchy of classes of formal grammars. This hierarchy of grammars was described by Noam Chomsky in 1956.^{[1]} It is also named after MarcelPaul Schützenberger, who played a crucial role in the development of the theory of formal languages.
Contents

Formal grammars 1

The hierarchy 2

References 3
Formal grammars
A formal grammar of this type consists of a finite set of production rules (lefthand side \rightarrow \, righthand side), where each side consists of a sequence of the following symbols:

a finite set of nonterminal symbols (indicating that some production rule can yet be applied)

a finite set of terminal symbols (indicating that no production rule can be applied)

a start symbol (a distinguished nonterminal symbol)
A formal grammar defines (or generates) a formal language, which is a (usually infinite) set of finitelength sequences of symbols (i.e. strings) that may be constructed by applying production rules to another sequence of symbols which initially contains just the start symbol. A rule may be applied to a sequence of symbols by replacing an occurrence of the symbols on the lefthand side of the rule with those that appear on the righthand side. A sequence of rule applications is called a derivation. Such a grammar defines the formal language: all words consisting solely of terminal symbols which can be reached by a derivation from the start symbol.
Nonterminals are often represented by uppercase letters, terminals by lowercase letters, and the start symbol by S. For example, the grammar with terminals \{a, b\}, nonterminals \{S, A, B\}, production rules

S \rightarrow \, ABS

S \rightarrow \, ε (where ε is the empty string)

BA \rightarrow \, AB

BS \rightarrow \, b

Bb \rightarrow \, bb

Ab \rightarrow \, ab

Aa \rightarrow \, aa
and start symbol S, defines the language of all words of the form a^n b^n (i.e. n copies of a followed by n copies of b). The following is a simpler grammar that defines the same language: Terminals \{a, b\}, Nonterminals \{S\}, Start symbol S, Production rules

S \rightarrow \, aSb

S \rightarrow \, ε
As another example, a grammar for a toy subset of English language is given by:
terminals \{ generate, hate, great, green, ideas, linguists \},
nonterminals \{\textit{SENTENCE}, \textit{NOUNPHRASE}, \textit{VERBPHRASE}, \textit{NOUN}, \textit{VERB}, \textit{ADJ} \},
production rules

\textit{SENTENCE} \rightarrow \, \textit{NOUNPHRASE} \; \textit{VERBPHRASE}

\textit{NOUNPHRASE} \rightarrow \, \textit{ADJ} \; \textit{NOUNPHRASE}

\textit{NOUNPHRASE} \rightarrow \, \textit{NOUN}

\textit{VERBPHRASE} \rightarrow \, \textit{VERB} \; \textit{NOUNPHRASE}

\textit{VERBPHRASE} \rightarrow \, \textit{VERB}

\textit{NOUN} \rightarrow \, \textit{ideas}

\textit{NOUN} \rightarrow \, \textit{linguists}

\textit{VERB} \rightarrow \, \textit{generate}

\textit{VERB} \rightarrow \, \textit{hate}

\textit{ADJ} \rightarrow \, \textit{great}

\textit{ADJ} \rightarrow \, \textit{green}
and start symbol \textit{SENTENCE}. An example derivation is

SENTENCE \rightarrow NOUNPHRASE VERBPHRASE \rightarrow ADJ NOUNPHRASE VERBPHRASE \rightarrow ADJ NOUN VERBPHRASE \rightarrow ADJ NOUN VERB NOUNPHRASE \rightarrow ADJ NOUN VERB ADJ NOUNPHRASE \rightarrow ADJ NOUN VERB ADJ ADJ NOUNPHRASE \rightarrow ADJ NOUN VERB ADJ ADJ NOUN \rightarrow great NOUN VERB ADJ ADJ NOUN \rightarrow great linguists VERB ADJ ADJ NOUN \rightarrow great linguists generate ADJ ADJ NOUN \rightarrow great linguists generate great ADJ NOUN \rightarrow great linguists generate great green NOUN \rightarrow great linguists generate great green ideas.
Other sequences that can be derived from this grammar are "ideas hate great linguists", and "ideas generate". While these sentences are nonsensical, they are syntactically correct. A syntactically incorrect sentence like e.g. "ideas ideas great hate" cannot be derived from this grammar. See "Colorless green ideas sleep furiously" for a similar example given by Chomsky in 1957; see Phrase structure grammar and Phrase structure rules for more naturallanguage examples and the problems of formal grammars in that area.
The hierarchy
Set inclusions described by the Chomsky hierarchy
The Chomsky hierarchy consists of the following levels:

Type0 grammars (unrestricted grammars) include all formal grammars. They generate exactly all languages that can be recognized by a Turing machine. These languages are also known as the recursively enumerable languages. Note that this is different from the recursive languages which can be decided by an alwayshalting Turing machine.

Type1 grammars (contextsensitive grammars) generate the contextsensitive languages. These grammars have rules of the form \alpha A\beta \rightarrow \alpha\gamma\beta with A a nonterminal and \alpha, \beta and \gamma strings of terminals and/or nonterminals. The strings \alpha and \beta may be empty, but \gamma must be nonempty. The rule S \rightarrow \epsilon is allowed if S does not appear on the right side of any rule. The languages described by these grammars are exactly all languages that can be recognized by a linear bounded automaton (a nondeterministic Turing machine whose tape is bounded by a constant times the length of the input.)

Type2 grammars (contextfree grammars) generate the contextfree languages. These are defined by rules of the form A \rightarrow \gamma with A a nonterminal and \gamma a string of terminals and/or nonterminals. These languages are exactly all languages that can be recognized by a nondeterministic pushdown automaton. Contextfree languages – or rather its subset of deterministic contextfree language – are the theoretical basis for the phrase structure of most programming languages, though their syntax also includes contextsensitive name resolution due to declarations and scope. Often a subset of grammars are used to make parsing easier, such as by an LL parser.

Type3 grammars (regular grammars) generate the regular languages. Such a grammar restricts its rules to a single nonterminal on the lefthand side and a righthand side consisting of a single terminal, possibly followed by a single nonterminal (right regular). Alternatively, the righthand side of the grammar can consist of a single terminal, possibly preceded by a single nonterminal (left regular); these generate the same languages – however, if leftregular rules and rightregular rules are combined, the language need no longer be regular. The rule S \rightarrow \epsilon is also allowed here if S does not appear on the right side of any rule. These languages are exactly all languages that can be decided by a finite state automaton. Additionally, this family of formal languages can be obtained by regular expressions. Regular languages are commonly used to define search patterns and the lexical structure of programming languages.
Note that the set of grammars corresponding to recursive languages is not a member of this hierarchy; these would be properly between Type0 and Type1.
Every regular language is contextfree, every contextfree language (not containing the empty string) is contextsensitive, every contextsensitive language is recursive and every recursive language is recursively enumerable. These are all proper inclusions, meaning that there exist recursively enumerable languages which are not contextsensitive, contextsensitive languages which are not contextfree and contextfree languages which are not regular.
Summary
The following table summarizes each of Chomsky's four types of grammars, the class of language it generates, the type of automaton that recognizes it, and the form its rules must have.
There are further categories of formal languages, some of which are given in the expandable navigation box at the bottom of this page.
References




Each category of languages, except those marked by a ^{*}, is a proper subset of the category directly above it. Any language in each category is generated by a grammar and by an automaton in the category in the same line.






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