Regular expression

Summary

A regular expression is a finite formula that describes a formal language, built from single letters with union, concatenation and the Kleene star. By Kleene's theorem, the languages described in this way are exactly the regular languages. Regular expressions give a compact text notation for patterns, and they are used in text editors, search tools and the lexical analysis of programming languages. Practical pattern languages often add features, such as back-references, that go beyond regular languages.