# Context-free language

In formal language theory, a **context-free language** (**CFL**) is a language generated by some context-free grammar (CFG). Different CF grammars can generate the same CF language. It is important to distinguish properties of the language (intrinsic properties) from properties of a particular grammar (extrinsic properties).

The set of all context-free languages is identical to the set of languages accepted by pushdown automata, which makes these languages amenable to parsing. Indeed, given a CFG, there is a direct way to produce a pushdown automaton for the grammar (and corresponding language), though going the other way (producing a grammar given an automaton) is not as direct.

Context-free languages have many applications in programming languages; for example, the language of all properly matched parentheses is generated by the grammar . Also, most arithmetic expressions are generated by context-free grammars.

## Examples

An archetypal context-free language is , the language of all non-empty even-length strings, the entire first halves of which are 's, and the entire second halves of which are 's. is generated by the grammar .
This language is not regular.
It is accepted by the pushdown automaton where is defined as follows:^{[note 1]}

Unambiguous CFLs are a proper subset of all CFLs: there are inherently ambiguous CFLs. An example of an inherently ambiguous CFL is the union of with . This set is context-free, since the union of two context-free languages is always context-free. But there is no way to unambiguously parse strings in the (non-context-free) subset which is the intersection of these two languages.Template:Sfn

## Languages that are not context-free

The set is a context-sensitive language, but there does not exist a context-free grammar generating this language.Template:Sfn So there exist context-sensitive languages which are not context-free. To prove that a given language is not context-free, one may employ the pumping lemma for context-free languages^{[1]} or a number of other methods, such as Ogden's lemma or Parikh's theorem.^{[2]}

## Closure properties

Context-free languages are closed under the following operations. That is, if *L* and *P* are context-free languages, the following languages are context-free as well:

- the union of
*L*and*P* - the reversal of
*L* - the concatenation of
*L*and*P* - the Kleene star of
*L* - the image of
*L*under a homomorphism - the image of
*L*under an inverse homomorphism - the cyclic shift of
*L*(the language )

Context-free languages are not closed under complement, intersection, or difference. However, if *L* is a context-free language and *D* is a regular language then both their intersection and their difference are context-free languages.

### Nonclosure under intersection and complement and difference

The context-free languages are not closed under intersection. This can be seen by taking the languages and , which are both context-free.^{[note 2]} Their intersection is , which can be shown to be non-context-free by the pumping lemma for context-free languages.

Context-free languages are also not closed under complementation, as for any languages A and B: .

Context-free language are also not closed under difference: L^{C} = Σ^{*} \ L

## Decidability properties

The following problems are undecidable for arbitrary context-free grammars A and B:

- Equivalence: Given two context-free grammars
*A*and*B*, is ? - Intersection Emptiness: Given two context-free grammars
*A*and*B*, is ? However, the intersection of a context-free language and a*regular*language is context-free,^{[3]}and the variant of the problem where*B*is a regular grammar is decidable. - Containment: Given a context-free grammar
*A*, is ? Again, the variant of the problem where*B*is a regular grammar is decidable. - Universality: Given a context-free grammar
*A*, is ?

The following problems are *decidable* for arbitrary context-free languages:

- Emptiness: Given a context-free grammar
*A*, is ? - Finiteness: Given a context-free grammar
*A*, is finite? - Membership: Given a context-free grammar
*G*, and a word , does ? Efficient polynomial-time algorithms for the membership problem are the CYK algorithm and Earley's Algorithm.

According to Hopcroft, Motwani, Ullman (2003),^{[4]}
many of the fundamental closure and (un)decidability properties of context-free languages were shown in the 1961 paper of Bar-Hillel, Perles, and Shamir^{[1]}

## Parsing

Determining an instance of the membership problem; i.e. given a string , determine whether where is the language generated by a given grammar ; is also known as *recognition*. Context-free recognition for Chomsky normal form grammars was shown by Leslie G. Valiant to be reducible to boolean matrix multiplication, thus inheriting its complexity upper bound of *O*(*n*^{2.3728639}).^{[5]}^{[6]}^{[note 3]}
Conversely, Lillian Lee has shown *O*(*n*^{3-ε}) boolean matrix multiplication to be reducible to *O*(*n*^{3-3ε}) CFG parsing, thus establishing some kind of lower bound for the latter.^{[7]}

Practical uses of context-free languages require also to produce a derivation tree that exhibits the structure that the grammar associates with the given string. The process of producing this tree is called *parsing*. Known parsers have a time complexity that is cubic in the size of the string that is parsed.

Formally, the set of all context-free languages is identical to the set of languages accepted by pushdown automata (PDA). Parser algorithms for context-free languages include the CYK algorithm and the Earley's Algorithm.

A special subclass of context-free languages are the deterministic context-free languages which are defined as the set of languages accepted by a deterministic pushdown automaton and can be parsed by a LR(k) parser.^{[8]}

See also parsing expression grammar as an alternative approach to grammar and parser.

## See also

## Notes

- ↑ meaning of 's arguments and results:
- ↑ A context-free grammar for the language
*A*is given by the following production rules, taking*S*as the start symbol:*S*→*Sc*|*aTb*|*ε*;*T*→*aTb*|*ε*. The grammar for*B*is analogous. - ↑ In Valiant's papers,
*O*(*n*^{2.81}) given, the then best known upper bound. See Matrix multiplication#Algorithms for efficient matrix multiplication and Coppersmith–Winograd algorithm for bound improvements since then.

## References

- ↑
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|CitationClass=book }} Chapter 2: Context-Free Languages, pp. 91–122.

- Jean-Michel Autebert, Jean Berstel, Luc Boasson, Context-Free Languages and Push-Down Automata, in: G. Rozenberg, A. Salomaa (eds.), Handbook of Formal Languages, Vol. 1, Springer-Verlag, 1997, 111-174.