Contextfree grammar
In formal language theory, a contextfree grammar (CFG) is a formal grammar whose production rules are of the form
 $A\ \to \ \alpha$
with $A$ a single nonterminal symbol, and $\alpha$ a string of terminals and/or nonterminals ($\alpha$ can be empty). A formal grammar is "context free" if its production rules can be applied regardless of the context of a nonterminal. No matter which symbols surround it, the single nonterminal on the left hand side can always be replaced by the right hand side. This is what distinguishes it from a contextsensitive grammar.
A formal grammar is essentially a set of production rules that describe all possible strings in a given formal language. Production rules are simple replacements. For example, the first rule in the picture,
 $\langle {\text{Stmt}}\rangle \to \langle {\text{Id}}\rangle =\langle {\text{Expr}}\rangle ;$
replaces $\langle {\text{Stmt}}\rangle$ with $\langle {\text{Id}}\rangle =\langle {\text{Expr}}\rangle ;$. There can be multiple replacement rules for a given nonterminal symbol. The language generated by a grammar is the set of all strings of terminal symbols that can be derived, by repeated rule applications, from some particular nonterminal symbol ("start symbol"). Nonterminal symbols are used during the derivation process, but do not appear in its final result string.
Languages generated by contextfree grammars are known as contextfree languages (CFL). Different contextfree grammars can generate the same contextfree language. It is important to distinguish the properties of the language (intrinsic properties) from the properties of a particular grammar (extrinsic properties). The language equality question (do two given contextfree grammars generate the same language?) is undecidable.
Contextfree grammars arise in linguistics where they are used to describe the structure of sentences and words in a natural language, and they were invented by the linguist Noam Chomsky for this purpose. By contrast, in computer science, as the use of recursivelydefined concepts increased, they were used more and more. In an early application, grammars are used to describe the structure of programming languages. In a newer application, they are used in an essential part of the Extensible Markup Language (XML) called the Document Type Definition.^{[2]}
In linguistics, some authors use the term phrase structure grammar to refer to contextfree grammars, whereby phrasestructure grammars are distinct from dependency grammars. In computer science, a popular notation for contextfree grammars is Backus–Naur form, or BNF.
Background
Since the time of Pāṇini, at least, linguists have described the grammars of languages in terms of their block structure, and described how sentences are recursively built up from smaller phrases, and eventually individual words or word elements. An essential property of these block structures is that logical units never overlap. For example, the sentence:
 John, whose blue car was in the garage, walked to the grocery store.
can be logically parenthesized (with the logical metasymbols [ ]) as follows:
 [John[, [whose [blue car]] [was [in [the garage]]],]] [walked [to [the [grocery store]]]].
A contextfree grammar provides a simple and mathematically precise mechanism for describing the methods by which phrases in some natural language are built from smaller blocks, capturing the "block structure" of sentences in a natural way. Its simplicity makes the formalism amenable to rigorous mathematical study. Important features of natural language syntax such as agreement and reference are not part of the contextfree grammar, but the basic recursive structure of sentences, the way in which clauses nest inside other clauses, and the way in which lists of adjectives and adverbs are swallowed by nouns and verbs, is described exactly.
Contextfree grammars are a special form of SemiThue systems that in their general form date back to the work of Axel Thue.
The formalism of contextfree grammars was developed in the mid1950s by Noam Chomsky,^{[3]} and also their classification as a special type of formal grammar (which he called phrasestructure grammars).^{[4]} Some authors, however, reserve the term for more restricted grammars in the Chomsky hierarchy: contextsensitive grammars or contextfree grammars. In a broader sense, phrase structure grammars are also known as constituency grammars. The defining trait of phrase structure grammars is thus their adherence to the constituency relation, as opposed to the dependency relation of dependency grammars. In Chomsky's generative grammar framework, the syntax of natural language was described by contextfree rules combined with transformation rules.^{[5]}
Block structure was introduced into computer programming languages by the Algol project (1957–1960), which, as a consequence, also featured a contextfree grammar to describe the resulting Algol syntax. This became a standard feature of computer languages, and the notation for grammars used in concrete descriptions of computer languages came to be known as Backus–Naur form, after two members of the Algol language design committee.^{[3]} The "block structure" aspect that contextfree grammars capture is so fundamental to grammar that the terms syntax and grammar are often identified with contextfree grammar rules, especially in computer science. Formal constraints not captured by the grammar are then considered to be part of the "semantics" of the language.
Contextfree grammars are simple enough to allow the construction of efficient parsing algorithms that, for a given string, determine whether and how it can be generated from the grammar. An Earley parser is an example of such an algorithm, while the widely used LR and LL parsers are simpler algorithms that deal only with more restrictive subsets of contextfree grammars.
Formal definitions
A contextfree grammar G is defined by the 4tuple $G=(V,\Sigma ,R,S)$, where^{[6]}
 V is a finite set; each element $v\in V$ is called a nonterminal character or a variable. Each variable represents a different type of phrase or clause in the sentence. Variables are also sometimes called syntactic categories. Each variable defines a sublanguage of the language defined by G.
 Σ is a finite set of terminals, disjoint from V, which make up the actual content of the sentence. The set of terminals is the alphabet of the language defined by the grammar G.
 R is a finite relation in $V\times (V\cup \Sigma )^{*}$, where the asterisk represents the Kleene star operation. The members of R are called the (rewrite) rules or productions of the grammar. (also commonly symbolized by a P)
 S is the start variable (or start symbol), used to represent the whole sentence (or program). It must be an element of V.
Production rule notation
A production rule in R is formalized mathematically as a pair $(\alpha ,\beta )\in R$, where $\alpha \in V$ is a nonterminal and $\beta \in (V\cup \Sigma )^{*}$ is a string of variables and/or terminals; rather than using ordered pair notation, production rules are usually written using an arrow operator with $\alpha$ as its left hand side and β as its right hand side: $\alpha \rightarrow \beta$.
It is allowed for β to be the empty string, and in this case it is customary to denote it by ε. The form $\alpha \rightarrow \varepsilon$ is called an εproduction.^{[7]}
It is common to list all righthand sides for the same lefthand side on the same line, using  (the vertical bar) to separate them. Rules $\alpha \rightarrow \beta _{1}$ and $\alpha \rightarrow \beta _{2}$ can hence be written as $\alpha \rightarrow \beta _{1}\mid \beta _{2}$. In this case, $\beta _{1}$ and $\beta _{2}$ are called the first and second alternative, respectively.
Rule application
For any strings $u,v\in (V\cup \Sigma )^{*}$, we say u directly yields v, written as $u\Rightarrow v\,$, if $\exists (\alpha ,\beta )\in R$ with $\alpha \in V$ and $u_{1},u_{2}\in (V\cup \Sigma )^{*}$ such that $u\,=u_{1}\alpha u_{2}$ and $v\,=u_{1}\beta u_{2}$. Thus, v is a result of applying the rule $(\alpha ,\beta )$ to u.
Repetitive rule application
For any strings $u,v\in (V\cup \Sigma )^{*},$ we say u yields v or v is derived from u if there is a positive integer k and strings $u_{1},\ldots ,u_{k}\in (V\cup \Sigma )^{*}$ such that $u=u_{1}\Rightarrow u_{2}\Rightarrow \cdots \Rightarrow u_{k}=v$. This relation is denoted $u{\stackrel {*}{\Rightarrow }}v$, or $u\Rightarrow \Rightarrow v$ in some textbooks. If $k\geq 2$, the relation $u{\stackrel {+}{\Rightarrow }}v$ holds. In other words, $({\stackrel {*}{\Rightarrow }})$ and $({\stackrel {+}{\Rightarrow }})$ are the reflexive transitive closure (allowing a string to yield itself) and the transitive closure (requiring at least one step) of $(\Rightarrow )$, respectively.
Contextfree language
The language of a grammar $G=(V,\Sigma ,R,S)$ is the set
 $L(G)=\{w\in \Sigma ^{*}:S{\stackrel {*}{\Rightarrow }}w\}$
of all terminalsymbol strings derivable from the start symbol.
A language L is said to be a contextfree language (CFL), if there exists a CFG G, such that $L\,=\,L(G)$.
Nondeterministic pushdown automata recognize exactly the contextfree languages.
Examples
Words concatenated with their reverse
The grammar $G=(\{S\},\{a,b\},P,S)$, with productions
 S → aSa,
 S → bSb,
 S → ε,
is contextfree. It is not proper since it includes an εproduction. A typical derivation in this grammar is
 S → aSa → aaSaa → aabSbaa → aabbaa.
This makes it clear that $L(G)=\{ww^{R}:w\in \{a,b\}^{*}\}$. The language is contextfree, however, it can be proved that it is not regular.
If the productions
 S → a,
 S → b,
are added, a contextfree grammar for the set of all palindromes over the alphabet { a, b } is obtained.^{[8]}
Wellformed parentheses
The canonical example of a contextfree grammar is parenthesis matching, which is representative of the general case. There are two terminal symbols "(" and ")" and one nonterminal symbol S. The production rules are
 S → SS,
 S → (S),
 S → ()
The first rule allows the S symbol to multiply; the second rule allows the S symbol to become enclosed by matching parentheses; and the third rule terminates the recursion.^{[9]}
Wellformed nested parentheses and square brackets
A second canonical example is two different kinds of matching nested parentheses, described by the productions:
 S → SS
 S → ()
 S → (S)
 S → []
 S → [S]
with terminal symbols [ ] ( ) and nonterminal S.
The following sequence can be derived in that grammar:
 ([ [ [ ()() [ ][ ] ] ]([ ]) ])
Matching pairs
In a contextfree grammar, we can pair up characters the way we do with brackets. The simplest example:
 S → aSb
 S → ab
This grammar generates the language $\{a^{n}b^{n}:n\geq 1\}$, which is not regular (according to the pumping lemma for regular languages).
The special character ε stands for the empty string. By changing the above grammar to
 S → aSb
 S → ε
we obtain a grammar generating the language $\{a^{n}b^{n}:n\geq 0\}$ instead. This differs only in that it contains the empty string while the original grammar did not.
Distinct number of a's and b's
A contextfree grammar for the language consisting of all strings over {a,b} containing an unequal number of a's and b's:
 S → T  U
 T → VaT  VaV  TaV
 U → VbU  VbV  UbV
 V → aVbV  bVaV  ε
Here, the nonterminal T can generate all strings with more a's than b's, the nonterminal U generates all strings with more b's than a's and the nonterminal V generates all strings with an equal number of a's and b's. Omitting the third alternative in the rules for T and U doesn't restrict the grammar's language.
Second block of b's of double size
Another example of a nonregular language is $\{{\text{b}}^{n}{\text{a}}^{m}{\text{b}}^{2n}:n\geq 0,m\geq 0\}$. It is contextfree as it can be generated by the following contextfree grammar:
 S → bSbb  A
 A → aA  ε
Firstorder logic formulas
The formation rules for the terms and formulas of formal logic fit the definition of contextfree grammar, except that the set of symbols may be infinite and there may be more than one start symbol.
Examples of languages that are not context free
In contrast to wellformed nested parentheses and square brackets in the previous section, there is no contextfree grammar for generating all sequences of two different types of parentheses, each separately balanced disregarding the other, where the two types need not nest inside one another, for example:
 [ ( ] )
or
 [ [ [ [(((( ] ] ] ]))))(([ ))(([ ))([ )( ])( ])( ])
The fact that this language is not context free can be proven using pumping lemma for contextfree languages and a proof by contradiction, observing that all words of the form ${(}^{n}{[}^{n}{)}^{n}{]}^{n}$ should belong to the language. This language belongs instead to a more general class and can be described by a conjunctive grammar, which in turn also includes other noncontextfree languages, such as the language of all words of the form ${\text{a}}^{n}{\text{b}}^{n}{\text{c}}^{n}$.
Regular grammars
Every regular grammar is contextfree, but not all contextfree grammars are regular.^{[10]} The following contextfree grammar, for example, is also regular.
 S → a
 S → aS
 S → bS
The terminals here are a and b, while the only nonterminal is S. The language described is all nonempty strings of $a$s and $b$s that end in $a$.
This grammar is regular: no rule has more than one nonterminal in its righthand side, and each of these nonterminals is at the same end of the righthand side.
Every regular grammar corresponds directly to a nondeterministic finite automaton, so we know that this is a regular language.
Using vertical bars, the grammar above can be described more tersely as follows:
 S → a  aS  bS
Derivations and syntax trees
A derivation of a string for a grammar is a sequence of grammar rule applications that transform the start symbol into the string. A derivation proves that the string belongs to the grammar's language.
A derivation is fully determined by giving, for each step:
 the rule applied in that step
 the occurrence of its lefthand side to which it is applied
For clarity, the intermediate string is usually given as well.
For instance, with the grammar:
 S → S + S
 S → 1
 S → a
the string
 1 + 1 + a
can be derived from the start symbol S with the following derivation:
 S
 → S + S (by rule 1. on S)
 → S + S + S (by rule 1. on the second S)
 → 1 + S + S (by rule 2. on the first S)
 → 1 + 1 + S (by rule 2. on the second S)
 → 1 + 1 + a (by rule 3. on the third S)
Often, a strategy is followed that deterministically chooses the next nonterminal to rewrite:
 in a leftmost derivation, it is always the leftmost nonterminal;
 in a rightmost derivation, it is always the rightmost nonterminal.
Given such a strategy, a derivation is completely determined by the sequence of rules applied. For instance, one leftmost derivation of the same string is
 S
 → S + S (by rule 1 on the leftmost S)
 → 1 + S (by rule 2 on the leftmost S)
 → 1 + S + S (by rule 1 on the leftmost S)
 → 1 + 1 + S (by rule 2 on the leftmost S)
 → 1 + 1 + a (by rule 3 on the leftmost S),
which can be summarized as
 rule 1
 rule 2
 rule 1
 rule 2
 rule 3.
One rightmost derivation is:
 S
 → S + S (by rule 1 on the rightmost S)
 → S + S + S (by rule 1 on the rightmost S)
 → S + S + a (by rule 3 on the rightmost S)
 → S + 1 + a (by rule 2 on the rightmost S)
 → 1 + 1 + a (by rule 2 on the rightmost S),
which can be summarized as
 rule 1
 rule 1
 rule 3
 rule 2
 rule 2.
The distinction between leftmost derivation and rightmost derivation is important because in most parsers the transformation of the input is defined by giving a piece of code for every grammar rule that is executed whenever the rule is applied. Therefore, it is important to know whether the parser determines a leftmost or a rightmost derivation because this determines the order in which the pieces of code will be executed. See for an example LL parsers and LR parsers.
A derivation also imposes in some sense a hierarchical structure on the string that is derived. For example, if the string "1 + 1 + a" is derived according to the leftmost derivation outlined above, the structure of the string would be:
 {{1}_{S} + {{1}_{S} + {a}_{S}}_{S}}_{S}
where {...}_{S} indicates a substring recognized as belonging to S. This hierarchy can also be seen as a tree:
This tree is called a parse tree or "concrete syntax tree" of the string, by contrast with the abstract syntax tree. In this case the presented leftmost and the rightmost derivations define the same parse tree; however, there is another rightmost derivation of the same string
 S
 → S + S (by rule 1 on the rightmost S)
 → S + a (by rule 3 on the rightmost S)
 → S + S + a (by rule 1 on the rightmost S)
 → S + 1 + a (by rule 2 on the rightmost S)
 → 1 + 1 + a (by rule 2 on the rightmost S),
which defines a string with a different structure
 {{{1}_{S} + {1}_{S}}_{S} + {a}_{S}}_{S}
and a different parse tree:
Note however that both parse trees can be obtained by both leftmost and rightmost derivations. For example, the last tree can be obtained with the leftmost derivation as follows:
 S
 → S + S (by rule 1 on the leftmost S)
 → S + S + S (by rule 1 on the leftmost S)
 → 1 + S + S (by rule 2 on the leftmost S)
 → 1 + 1 + S (by rule 2 on the leftmost S)
 → 1 + 1 + a (by rule 3 on the leftmost S),
If a string in the language of the grammar has more than one parsing tree, then the grammar is said to be an ambiguous grammar. Such grammars are usually hard to parse because the parser cannot always decide which grammar rule it has to apply. Usually, ambiguity is a feature of the grammar, not the language, and an unambiguous grammar can be found that generates the same contextfree language. However, there are certain languages that can only be generated by ambiguous grammars; such languages are called inherently ambiguous languages.
Example: Algebraic expressions
Here is a contextfree grammar for syntactically correct infix algebraic expressions in the variables x, y and z:
 S → x
 S → y
 S → z
 S → S + S
 S → S – S
 S → S * S
 S → S / S
 S → (S)
This grammar can, for example, generate the string
 (x + y) * x – z * y / (x + x)
as follows:
 S
 → S – S (by rule 5)
 → S * S – S (by rule 6, applied to the leftmost S)
 → S * S – S / S (by rule 7, applied to the rightmost S)
 → (S) * S – S / S (by rule 8, applied to the leftmost S)
 → (S) * S – S / (S) (by rule 8, applied to the rightmost S)
 → (S + S) * S – S / (S) (by rule 4, applied to the leftmost S)
 → (S + S) * S – S * S / (S) (by rule 6, applied to the fourth S)
 → (S + S) * S – S * S / (S + S) (by rule 4, applied to the rightmost S)
 → (x + S) * S – S * S / (S + S) (etc.)
 → (x + y) * S – S * S / (S + S)
 → (x + y) * x – S * S / (S + S)
 → (x + y) * x – z * S / (S + S)
 → (x + y) * x – z * y / (S + S)
 → (x + y) * x – z * y / (x + S)
 → (x + y) * x – z * y / (x + x)
Note that many choices were made underway as to which rewrite was going to be performed next. These choices look quite arbitrary. As a matter of fact, they are, in the sense that the string finally generated is always the same. For example, the second and third rewrites
 → S * S – S (by rule 6, applied to the leftmost S)
 → S * S – S / S (by rule 7, applied to the rightmost S)
could be done in the opposite order:
 → S – S / S (by rule 7, applied to the rightmost S)
 → S * S – S / S (by rule 6, applied to the leftmost S)
Also, many choices were made on which rule to apply to each selected S. Changing the choices made and not only the order they were made in usually affects which terminal string comes out at the end.
Let's look at this in more detail. Consider the parse tree of this derivation:
Starting at the top, step by step, an S in the tree is expanded, until no more unexpanded Ses (nonterminals) remain. Picking a different order of expansion will produce a different derivation, but the same parse tree. The parse tree will only change if we pick a different rule to apply at some position in the tree.
But can a different parse tree still produce the same terminal string, which is (x + y) * x – z * y / (x + x) in this case? Yes, for this particular grammar, this is possible. Grammars with this property are called ambiguous.
For example, x + y * z can be produced with these two different parse trees:
However, the language described by this grammar is not inherently ambiguous: an alternative, unambiguous grammar can be given for the language, for example:
 T → x
 T → y
 T → z
 S → S + T
 S → S – T
 S → S * T
 S → S / T
 T → (S)
 S → T,
once again picking S as the start symbol. This alternative grammar will produce x + y * z with a parse tree similar to the left one above, i.e. implicitly assuming the association (x + y) * z, which does not follow standard order of operations. More elaborate, unambiguous and contextfree grammars can be constructed that produce parse trees that obey all desired operator precedence and associativity rules.
Normal forms
Every contextfree grammar with no εproduction has an equivalent grammar in Chomsky normal form, and a grammar in Greibach normal form. "Equivalent" here means that the two grammars generate the same language.
The especially simple form of production rules in Chomsky normal form grammars has both theoretical and practical implications. For instance, given a contextfree grammar, one can use the Chomsky normal form to construct a polynomialtime algorithm that decides whether a given string is in the language represented by that grammar or not (the CYK algorithm).
Closure properties
Contextfree languages are closed under the various operations, that is, if the languages K and L are contextfree, so is the result of the following operations:
 union K ∪ L; concatenation K ∘ L; Kleene star L^{*}^{[11]}
 substitution (in particular homomorphism)^{[12]}
 inverse homomorphism^{[13]}
 intersection with a regular language^{[14]}
They are not closed under general intersection (hence neither under complementation) and set difference.^{[15]}
Decidable problems
The following are some decidable problems about contextfree grammars.
Parsing
The parsing problem, checking whether a given word belongs to the language given by a contextfree grammar, is decidable, using one of the generalpurpose parsing algorithms:
 CYK algorithm (for grammars in Chomsky normal form)
 Earley parser
 GLR parser
 LL parser (only for the proper subclass of for LL(k) grammars)
Contextfree parsing 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}).^{[16]}^{[17]}^{[note 1]} 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.^{[18]}
Reachability, productiveness, nullability
Example grammar:  

S → Bb  Cc  Ee  
B → Bb  b  
C → C  
D → Bd  Cd  d  
E → Ee 
A nonterminal symbol $X$ is called productive, or generating, if there is a derivation $X{\stackrel {*}{\Rightarrow }}w$ for some string $w$ of terminal symbols. $X$ is called reachable if there is a derivation $S{\stackrel {*}{\Rightarrow }}\alpha X\beta$ for some strings $\alpha ,\beta$ of nonterminal and terminal symbols from the start symbol. $X$ is called useless if it is unreachable or unproductive. $X$ is called nullable if there is a derivation $X{\stackrel {*}{\Rightarrow }}\varepsilon$. A rule $X\rightarrow \varepsilon$ is called an εproduction. A derivation $X{\stackrel {+}{\Rightarrow }}X$ is called a cycle.
Algorithms are known to eliminate from a given grammar, without changing its generated language,
 unproductive symbols,^{[19]}^{[note 2]}
 unreachable symbols,^{[21]}^{[22]}^{[23]}
 εproductions, with one possible exception,^{[note 3]}^{[24]} and
 cycles.^{[note 4]}
In particular, an alternative containing a useless nonterminal symbol can be deleted from the righthand side of a rule. Such rules and alternatives are called useless.^{[25]}
In the depicted example grammar, the nonterminal D is unreachable, and E is unproductive, while C → C causes a cycle. Hence, omitting the last three rules doesn't change the language generated by the grammar, nor does omitting the alternatives " Cc  Ee" from the righthand side of the rule for S.
A contextfree grammar is said to be proper if it has neither useless symbols nor εproductions nor cycles.^{[26]} Combining the above algorithms, every contextfree grammar not generating ε can be transformed into a weakly equivalent proper one.
Regularity and LL(k) checks
It is decidable whether a given grammar is a regular grammar,^{[27]} as well as whether it is an LL(k) grammar for a given k≥0.^{[28]}^{: 233 } If k is not given, the latter problem is undecidable.^{[28]}^{: 252 }
Given a contextfree language, it is neither decidable whether it is regular,^{[29]} nor whether it is an LL(k) language for a given k.^{[28]}^{: 254 }
Emptiness and finiteness
There are algorithms to decide whether a language of a given contextfree language is empty, as well as whether it is finite.^{[30]}
Undecidable problems
Some questions that are undecidable for wider classes of grammars become decidable for contextfree grammars; e.g. the emptiness problem (whether the grammar generates any terminal strings at all), is undecidable for contextsensitive grammars, but decidable for contextfree grammars.
However, many problems are undecidable even for contextfree grammars. Examples are:
Universality
Given a CFG, does it generate the language of all strings over the alphabet of terminal symbols used in its rules?^{[31]}^{[32]}
A reduction can be demonstrated to this problem from the wellknown undecidable problem of determining whether a Turing machine accepts a particular input (the halting problem). The reduction uses the concept of a computation history, a string describing an entire computation of a Turing machine. A CFG can be constructed that generates all strings that are not accepting computation histories for a particular Turing machine on a particular input, and thus it will accept all strings only if the machine doesn't accept that input.
Language equality
Given two CFGs, do they generate the same language?^{[32]}^{[33]}
The undecidability of this problem is a direct consequence of the previous: it is impossible to even decide whether a CFG is equivalent to the trivial CFG defining the language of all strings.
Language inclusion
Given two CFGs, can the first one generate all strings that the second one can generate?^{[32]}^{[33]}
If this problem was decidable, then language equality could be decided too: two CFGs G1 and G2 generate the same language if L(G1) is a subset of L(G2) and L(G2) is a subset of L(G1).
Being in a lower or higher level of the Chomsky hierarchy
Using Greibach's theorem, it can be shown that the two following problems are undecidable:
 Given a contextsensitive grammar, does it describe a contextfree language?
 Given a contextfree grammar, does it describe a regular language?^{[32]}^{[33]}
Grammar ambiguity
Given a CFG, is it ambiguous?
The undecidability of this problem follows from the fact that if an algorithm to determine ambiguity existed, the Post correspondence problem could be decided, which is known to be undecidable.
Language disjointness
Given two CFGs, is there any string derivable from both grammars?
If this problem was decidable, the undecidable Post correspondence problem could be decided, too: given strings $\alpha _{1},\ldots ,\alpha _{N},\beta _{1},\ldots ,\beta _{N}$ over some alphabet $\{a_{1},\ldots ,a_{k}\}$, let the grammar $G_{1}$ consist of the rule
 $S\to \alpha _{1}S\beta _{1}^{rev}\cdots \alpha _{N}S\beta _{N}^{rev}b$;
where $\beta _{i}^{rev}$ denotes the reversed string $\beta _{i}$ and $b$ doesn't occur among the $a_{i}$; and let grammar $G_{2}$ consist of the rule
 $T\to a_{1}Ta_{1}\cdots a_{k}Ta_{k}b$;
Then the Post problem given by $\alpha _{1},\ldots ,\alpha _{N},\beta _{1},\ldots ,\beta _{N}$ has a solution if and only if $L(G_{1})$ and $L(G_{2})$ share a derivable string.
Extensions
An obvious way to extend the contextfree grammar formalism is to allow nonterminals to have arguments, the values of which are passed along within the rules. This allows natural language features such as agreement and reference, and programming language analogs such as the correct use and definition of identifiers, to be expressed in a natural way. E.g. we can now easily express that in English sentences, the subject and verb must agree in number. In computer science, examples of this approach include affix grammars, attribute grammars, indexed grammars, and Van Wijngaarden twolevel grammars. Similar extensions exist in linguistics.
An extended contextfree grammar (or regular right part grammar) is one in which the righthand side of the production rules is allowed to be a regular expression over the grammar's terminals and nonterminals. Extended contextfree grammars describe exactly the contextfree languages.^{[34]}
Another extension is to allow additional terminal symbols to appear at the lefthand side of rules, constraining their application. This produces the formalism of contextsensitive grammars.
Subclasses
There are a number of important subclasses of the contextfree grammars:
 LR(k) grammars (also known as deterministic contextfree grammars) allow parsing (string recognition) with deterministic pushdown automata (PDA), but they can only describe deterministic contextfree languages.
 Simple LR, LookAhead LR grammars are subclasses that allow further simplification of parsing. SLR and LALR are recognized using the same PDA as LR, but with simpler tables, in most cases.
 LL(k) and LL(*) grammars allow parsing by direct construction of a leftmost derivation as described above, and describe even fewer languages.
 Simple grammars are a subclass of the LL(1) grammars mostly interesting for its theoretical property that language equality of simple grammars is decidable, while language inclusion is not.
 Bracketed grammars have the property that the terminal symbols are divided into left and right bracket pairs that always match up in rules.
 Linear grammars have no rules with more than one nonterminal on the righthand side.
 Regular grammars are a subclass of the linear grammars and describe the regular languages, i.e. they correspond to finite automata and regular expressions.
LR parsing extends LL parsing to support a larger range of grammars; in turn, generalized LR parsing extends LR parsing to support arbitrary contextfree grammars. On LL grammars and LR grammars, it essentially performs LL parsing and LR parsing, respectively, while on nondeterministic grammars, it is as efficient as can be expected. Although GLR parsing was developed in the 1980s, many new language definitions and parser generators continue to be based on LL, LALR or LR parsing up to the present day.
Linguistic applications
Chomsky initially hoped to overcome the limitations of contextfree grammars by adding transformation rules.^{[4]}
Such rules are another standard device in traditional linguistics; e.g. passivization in English. Much of generative grammar has been devoted to finding ways of refining the descriptive mechanisms of phrasestructure grammar and transformation rules such that exactly the kinds of things can be expressed that natural language actually allows. Allowing arbitrary transformations does not meet that goal: they are much too powerful, being Turing complete unless significant restrictions are added (e.g. no transformations that introduce and then rewrite symbols in a contextfree fashion).
Chomsky's general position regarding the noncontextfreeness of natural language has held up since then,^{[35]} although his specific examples regarding the inadequacy of contextfree grammars in terms of their weak generative capacity were later disproved.^{[36]} Gerald Gazdar and Geoffrey Pullum have argued that despite a few noncontextfree constructions in natural language (such as crossserial dependencies in Swiss German^{[35]} and reduplication in Bambara^{[37]}), the vast majority of forms in natural language are indeed contextfree.^{[36]}
See also
 Parsing expression grammar
 Stochastic contextfree grammar
 Algorithms for contextfree grammar generation
 Pumping lemma for contextfree languages
References
 ^ Brian W. Kernighan and Dennis M. Ritchie (Apr 1988). The C Programming Language. Prentice Hall Software Series (2nd ed.). Englewood Cliffs/NJ: Prentice Hall. ISBN 0131103628. Here: App.A
 ^ Introduction to Automata Theory, Languages, and Computation, John E. Hopcroft, Rajeev Motwani, Jeffrey D. Ullman, Addison Wesley, 2001, p.191
 ^ ^{a} ^{b} Hopcroft & Ullman (1979), p. 106.
 ^ ^{a} ^{b} Chomsky, Noam (Sep 1956), "Three models for the description of language", IEEE Transactions on Information Theory, 2 (3): 113–124, doi:10.1109/TIT.1956.1056813
 ^ https://web.stanford.edu/~jurafsky/slp3/12.pdf^{[bare URL PDF]}
 ^ The notation here is that of Sipser (1997), p. 94. Hopcroft & Ullman (1979) (p. 79) define contextfree grammars as 4tuples in the same way, but with different variable names.
 ^ Hopcroft & Ullman (1979), pp. 90–92.
 ^ Hopcroft & Ullman (1979), Exercise 4.1a, p. 103.
 ^ Hopcroft & Ullman (1979), Exercise 4.1b, p. 103.
 ^ Aho, Alfred Vaino; Lam, Monica S.; Sethi, Ravi; Ullman, Jeffrey David (2007). "4.2.7 ContextFree Grammars Versus Regular Expressions" (print). Compilers: Principles, Techniques, & Tools (2nd ed.). Boston, MA USA: Pearson AddisonWesley. pp. 205–206. ISBN 9780321486813.
Every construct that can be described by a regular expression can be described by a [contextfree] grammar, but not viceversa.
 ^ Hopcroft & Ullman (1979), p.131, Theorem 6.1
 ^ Hopcroft & Ullman (1979), pp.131–132, Theorem 6.2
 ^ Hopcroft & Ullman (1979), pp.132–134, Theorem 6.3
 ^ Hopcroft & Ullman (1979), pp.135–136, Theorem 6.5
 ^ Hopcroft & Ullman (1979), pp.134–135, Theorem 6.4
 ^ Leslie Valiant (Jan 1974). General contextfree recognition in less than cubic time (Technical report). Carnegie Mellon University. p. 11.
 ^ Leslie G. Valiant (1975). "General contextfree recognition in less than cubic time". Journal of Computer and System Sciences. 10 (2): 308–315. doi:10.1016/s00220000(75)800468.
 ^ Lillian Lee (2002). "Fast ContextFree Grammar Parsing Requires Fast Boolean Matrix Multiplication" (PDF). J ACM. 49 (1): 1–15. arXiv:cs/0112018. doi:10.1145/505241.505242. S2CID 1243491.
 ^ Hopcroft & Ullman (1979), Lemma 4.1, p. 88.
 ^ Aiken, A.; Murphy, B. (1991). "Implementing Regular Tree Expressions". ACM Conference on Functional Programming Languages and Computer Architecture. pp. 427–447. CiteSeerX 10.1.1.39.3766.; here: Sect.4
 ^ Hopcroft & Ullman (1979), Lemma 4.2, p. 89.
 ^ Hopcroft, Motwani & Ullman (2003) harvtxt error: no target: CITEREFHopcroftMotwaniUllman2003 (help), Theorem 7.2, Sect.7.1, p.255ff
 ^ (PDF) https://www.springer.com/cda/content/document/cda_downloaddocument/9780387202488c1.pdf?SGWID=0045466216p52091986.
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(help)  ^ Hopcroft & Ullman (1979), Theorem 4.3, p. 90.
 ^ John E. Hopcroft; Rajeev Motwani; Jeffrey D. Ullman (2003). Introduction to Automata Theory, Languages, and Computation. Addison Wesley.; here: Sect.7.1.1, p.256
 ^ Nijholt, Anton (1980), Contextfree grammars: covers, normal forms, and parsing, Lecture Notes in Computer Science, vol. 93, Springer, p. 8, ISBN 9783540102458, MR 0590047.
 ^ This is easy to see from the grammar definitions.
 ^ ^{a} ^{b} ^{c} D.J. Rosenkrantz and R.E. Stearns (1970). "Properties of Deterministic Top Down Grammars". Information and Control. 17 (3): 226–256. doi:10.1016/S00199958(70)904468.
 ^ Hopcroft & Ullman (1979), Exercise 8.10a, p. 214. The problem remains undecidable even if the language is produced by a "linear" contextfree grammar (i.e., with at most one nonterminal in each rule's righthand side, cf. Exercise 4.20, p. 105).
 ^ Hopcroft & Ullman (1979), pp.137–138, Theorem 6.6
 ^ Sipser (1997), Theorem 5.10, p. 181.
 ^ ^{a} ^{b} ^{c} ^{d} Hopcroft & Ullman (1979), p. 281.
 ^ ^{a} ^{b} ^{c} Hazewinkel, Michiel (1994), Encyclopaedia of mathematics: an updated and annotated translation of the Soviet "Mathematical Encyclopaedia", Springer, Vol. IV, p. 56, ISBN 9781556080036.
 ^ Norvell, Theodore. "A Short Introduction to Regular Expressions and ContextFree Grammars" (PDF). p. 4. Retrieved August 24, 2012.
 ^ ^{a} ^{b} Shieber, Stuart (1985), "Evidence against the contextfreeness of natural language" (PDF), Linguistics and Philosophy, 8 (3): 333–343, doi:10.1007/BF00630917, S2CID 222277837.
 ^ ^{a} ^{b} Pullum, Geoffrey K.; Gerald Gazdar (1982), "Natural languages and contextfree languages", Linguistics and Philosophy, 4 (4): 471–504, doi:10.1007/BF00360802, S2CID 189881482.
 ^ Culy, Christopher (1985), "The Complexity of the Vocabulary of Bambara", Linguistics and Philosophy, 8 (3): 345–351, doi:10.1007/BF00630918, S2CID 189881984.
Notes
 ^ In Valiant's papers, O(n^{2.81}) is given, the then best known upper bound. See Matrix multiplication#Computational complexity for bound improvements since then.
 ^ For regular tree grammars, Aiken and Murphy give a fixpoint algorithm to detect unproductive nonterminals.^{[20]}
 ^ If the grammar can generate $\varepsilon$, a rule $S\rightarrow \varepsilon$ cannot be avoided.
 ^ This is a consequence of the unitproduction elimination theorem in Hopcroft & Ullman (1979), p.91, Theorem 4.4
Further reading
 Hopcroft, John E.; Ullman, Jeffrey D. (1979), Introduction to Automata Theory, Languages, and Computation, AddisonWesley. Chapter 4: ContextFree Grammars, pp. 77–106; Chapter 6: Properties of ContextFree Languages, pp. 125–137.
 Sipser, Michael (1997), Introduction to the Theory of Computation, PWS Publishing, ISBN 9780534947286. Chapter 2: ContextFree Grammars, pp. 91–122; Section 4.1.2: Decidable problems concerning contextfree languages, pp. 156–159; Section 5.1.1: Reductions via computation histories: pp. 176–183.
 J. Berstel, L. Boasson (1990). Jan van Leeuwen (ed.). ContextFree Languages. Handbook of Theoretical Computer Science. Vol. B. Elsevier. pp. 59–102.
External links
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