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Some authors call this type of grammar a right-regular grammar (or right-linear grammar) [1] and the type above a strictly right-regular grammar (or strictly right-linear grammar). [2] An extended left-regular grammar is one in which all rules obey one of A → w, where A is a non-terminal in N and w is in Σ * A → Bw, where A and B are in N ...
In theoretical computer science and formal language theory, a regular language (also called a rational language) [1] [2] is a formal language that can be defined by a regular expression, in the strict sense in theoretical computer science (as opposed to many modern regular expression engines, which are augmented with features that allow the recognition of non-regular languages).
Automata also appear in the theory of finite fields: the set of irreducible polynomials that can be written as composition of degree two polynomials is in fact a regular language. [15] Another problem for which automata can be used is the induction of regular languages .
Successor automata can learn exactly the class of local languages. Since each regular language is the homomorphic image of a local language, grammars from the former class can be learned by lifting, if an appropriate (depending on the intended application) homomorphism is provided.
Regular languages are commonly used to define search patterns and the lexical structure of programming languages. For example, the regular language = {| >} is generated by the Type-3 grammar = ({}, {,},,) with the productions being the following. S → aS S → a
In particular, the minimum automaton of a star-free language is always counter-free (however, a star-free language may also be recognized by other automata that are not aperiodic). A counter-free language is a regular language for which there is an integer n such that for all words x, y, z and integers m ≥ n we have xy m z in L if and only if ...
It is decidable whether a given grammar is a regular grammar, [f] as well as whether it is an LL grammar for a given k≥0. [26]: 233 If k is not given, the latter problem is undecidable. [26]: 252 Given a context-free grammar, it is not decidable whether its language is regular, [27] nor whether it is an LL(k) language for a given k.
The PDA accepts by empty stack. Its initial stack symbol is the grammar's start symbol. [3] For a context-free grammar in Greibach normal form, defining (1,γ) ∈ δ(1,a,A) for each grammar rule A → aγ also yields an equivalent nondeterministic pushdown automaton. [4] The converse, finding a grammar for a given PDA, is not that easy.