where Λ is the empty string. A language is regular if and only if it is representable by a regular expression. Thus the class of regular languages is the smallest one that contains all finite languages and is closed under concatenation, union, and star – the so-called regular operations. These three operations correspond to “sequence”, “choice”, and “iteration” in structured iterative programs.
"regular expression." A Dictionary of Computing. . Encyclopedia.com. (August 18, 2018). http://www.encyclopedia.com/computing/dictionaries-thesauruses-pictures-and-press-releases/regular-expression
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