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{math, number, function} 
{language, word, form} 

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:
Contents
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 A ∪ B (union), A • B (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 .
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