context-free grammar




The notion of context-free grammar, now used in linguistics, computer science and mathematics, was introduced in the works of Noam Chomsky.

Traditional definition

We write X+YX + Y for the disjoint union of two sets and use vector notation and the Kleene star vX \vec{v} \in X^\star to denote sequences in the free monoid.

A context-free grammar is a tuple (Σ,X,R,s)(\Sigma, X, R, s) where Σ\Sigma and XX are finite sets called the vocabulary (also called the terminals) and the non-terminals respectively, RX×(X+Σ) R \subseteq X \times (X + \Sigma)^\star is a finite set of production rules and sXs \in X is called the start symbol.

The language of a context-free grammar is given by L(G)={uV |s Ru}L(G) = \{ \vec{u} \in V^\star \vert s \to_R \vec{u} \} where the rewriting relation ( R)(V+X) ×(V+X) (\to_R) \subseteq (V + X)^\star \times (V + X)^\star is traditionally defined as the transitive closure of the following directed graph:

{(uxw,uvw)|u,w(V+X) ,(x,v)R} \big\{ (\vec{u} x \vec{w}, \vec{u v w}) \quad \vert \quad \vec{u}, \vec{w} \in (V + X)^\star, (x, \vec{v}) \in R \big\}

With string diagrams

One may redefine L(G)={uΣ |C G(s,u)}L(G) = \big\{ \vec{u} \in \Sigma^\star \vert \C_G(s, \vec{u}) \neq \emptyset \big\} where C GC_G is the free monoidal category with:

  • generating objects the disjoint union Σ+X\Sigma + X,
  • generating arrows the production rules (x,v)R(x, \vec{v}) \in R with dom(x,v)=xdom(x, \vec{v}) = x and cod(x,v)=vcod(x, \vec{v}) = \vec{v}.

That is, a string uΣ \vec{u} \in \Sigma^\star is grammatical whenever there exists an arrow from the start symbol ss to u\vec{u} in C GC_G. Arrows in C GC_G may be encoded as a syntax tree, seen as a special case of a string diagram, e.g.:

syntax tree for Colorless green ideas sleep furiously

Note that context-free grammar is weakly equivalent to Lambek's pregroup grammar i.e. they generate the same class of languages, see:

  • Wojciech Buszkowski, Katarzyna Moroz, Pregroup Grammars and Context-free Grammars, Computational Algebraic Approaches to Natural Language, Polimetrica (2008) (pdf)

Early sources

Here is list of some of the original references in English and also of their Russian translations.

  • N. Chomsky, Three models for the description of language, I. R. E. Trans. PGIT 2 (1956), 113—124. (Русский перевод: Хомский Н., Три модели для описания языка, Кибернетический сборник, вып. 2, ИЛ 1961, 237-266)
  • N. Chomsky, On certain formal properties of grammars, Information and Control 2 (1959), 137—267; A note on phrase structure grammars, Information andControl 2 1959_, 393—395. (Хомский К, Заметки o грамматиках непосредственных составляющих. Кибернетический сборник, вып. 5, ИЛ, 1962, 312—315.)
  • N. Chomsky, On the notion «Rule of Grammar», Proc. Symp. Applied Math., 12. Amer. Math. Soc. (1961). (Хомский Н., О понятии «правило грамматики», сб. Новое в лингвистике, вып. 4, «Прогресс», 1965, стр. 34—65.)
  • N. Chomsky, Context-free grammars and pushdown storage, Quarterly Progress Reports, № 65, Research Laboratory of Electronics, M. I. T., 1962.
  • N. Chomsky, Formal properties of grammars, in Handbook of Mathemati- Mathematical Psychology, 2, ch. 12, Wiley, 1963, p. 323—418. (Хомский Н., Формальные свойства грамматик, Кибернетический сборник, вып. 2, «Мир», 1966, 121—-230.)
  • N. Chomsky, The logical basis for linguistic theory, Proc. IX-th Int. Cong. Linguists (1962). (Хомский Н“ Логические основы лингвистической теории, сб. Новое в лингвистике, вып. 4, «Прогресс», 1965, 465—-575.)
  • N. Сhоmskу, G. A. Miller G. A.. Finite state languages, Information and Control 1 (1958), 91—-112. (Хомский Н., Миллер Д., Языки с конечным числом состояний, Кибернетический сборник, вып. 4, ИЛ, 1962, 231—-255.), Introduction to the formal analysis of natural languages. Handbook of Mathematical Psychology 2, Ch. 12, Wiley, 1963, 269—-322 (Введение в формальный анализ естественных языков, Кибернетический сборник, вып. 1, «Мир», 1965, стр. 229—-290.)
  • N. Chomsky, M. P. Schützenberger, The algebraic theory of context-free languages, in: Computer programming and formal systems, 118–161 North-Holland 1963, Amsterdam MR152391 (Кибернетический сборник 3, 195–242, 1966)

Modern literature

  • wikipedia: context-free language
  • A. V. Aho, J. D. Ullman, The theory of parsing, translation, and compiling, vol 1, Parsing; vol. 2, Compiling. Prentice Hall, 1972.
  • A. V. Aho, J. D. Ullman, Principles of compiler design, Addison-Wesley, 1977.

Some chapters in “Handbook of formal language theory” (3 vols.), G. Rozenberg, A. Salomaa (eds.), Springer 1997:

  • Jean-Michel Autebert, Jean Berstel, Luc Boasson, Context-free languages and push-down automata, vol. 1, ch. 3

Last revised on December 1, 2020 at 08:33:18. See the history of this page for a list of all contributions to it.