A regular expression is built from alphabet symbols, the empty language and the empty word using finite union, concatenation and Kleene star.
The Kleene star of a language consists of every finite concatenation of words from , including the empty concatenation.
This theorem characterizes a regular language by regular expressions and acceptance by a finite-state automaton. Kleene's theorem states that a language is denoted by a regular expression exactly when it is accepted by a finite automaton.
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A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. It is primarily used for string matching and manipulation tasks in various programming languages and tools. Regular expressions can be used to identify, search, edit, or replace text based on specific patterns.
