# Regular language

## Formal language that can be expressed using a regular expression / From Wikipedia, the free encyclopedia

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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 expressions engines, which are augmented with features that allow recognition of non-regular languages).

Alternatively, a regular language can be defined as a language recognized by a finite automaton. The equivalence of regular expressions and finite automata is known as **Kleene's theorem**[3] (after American mathematician Stephen Cole Kleene). In the Chomsky hierarchy, regular languages are the languages generated by Type-3 grammars.