A regular language is a formal language (i.e. a possibly infinite set of finite sequences of symbols from a finite alphabet) that satisfies the following equivalent properties:
- it can be accepted by a deterministic finite state automaton.
- it can be accepted by a non-deterministic finite state automaton
- it can be described by a regular expression.
- it can be generated by a regular grammar
- it can be accepted by a read-only Turing machine
The result of the union, intersection and set-difference operations when applied to regular languages is itself a regular language; the complement of every regular language is a regular language as well. Reversing every string in a regular language yields another regular language. Concatenating two regular languages (in the sense of concatenating every string from the first language with every string from the second one) also yields a regular language. The shuffle operation, when applied to two regular languages, yields another regular language. The right quotient and the left quotient of a regular language by an arbitrary language is also regular.
To locate the regular languages in the Chomsky hierarchy, one notices that every regular language is context-free. The converse is not true: for example the language consisting of all strings having the same number of a's as b's is context-free but not regular. To prove that a language such as this is not regular, one uses the Myhill-Nerode Theorem or the pumping lemma.
There are two purely algebraic approaches to defining regular languages. If Σ is a finite alphabet and Σ* denotes the free monoid over Σ consisting of all strings over Σ, f : Σ* -> M is a monoid homomorphism where M is a finite monoid, and S is a subset of M, then the set f -1(S) is regular. Every regular language arises in this fashion.
If L is any subset of Σ*, one defines an equivalence relation ~ on Σ* as follows: u ~ v is defined to mean
- uw in L if and only if vw in L for all w in Σ*