Regular language

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In theoretical computer science, 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:


Regular languages

The collection of regular languages over an alphabet Σ is defined recursively as follows:

  • the empty language Ø is a regular language.
  • the empty string language { ε } is a regular language.
  • For each a ∈ Σ (a belongs to Σ ), the singleton language { a } is a regular language.
  • If A and B are regular languages, then AB (union), AB (concatenation), and A* (Kleene star) are regular languages.
  • No other languages over Σ are regular.

All finite languages are regular. Other typical examples include the language consisting of all strings over the alphabet {a, b} which contain an even number of as, or the language consisting of all strings of the form: several as followed by several bs.

A simple example of a language that is not regular is the set of strings \{a^nb^n\,\vert\; n\ge 0\}.

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