Kleene star

The Kleene star (or Kleene closure) is an operation used in regular expressions and operates either on sets of strings or on sets of symbols or characters. The application of the Kleene star to a set V is written as V*.

If V is a set of strings then V* is defined as the smallest superset of V that contains ε (the empty string) and is closed under the string concatenation operation. This set can also be described as the set of strings that can be made by concatenating zero or more strings from V.
If V is a set of symbols or characters then V* is the set of all strings over symbols in V, including the empty string.

Example of Kleene star applied to set of strings:

{"ab", "c"}* = {ε, "ab", "c", "abab", "abc", "cab", "cc", "ababab", "ababc", "abcab", "abcc", "cabab", "cabc", "ccab", "ccc", ...}

Example of Kleene star applied to set of characters:

{'a', 'b', 'c'}* = {ε, "a", "b", "c", "aa", "ab", "ac", "ba", "bb", "bc", ...}

The Kleene star is often generalized for any monoid (M, .), that is, a set M and binary operation '.' on M such that

(closure) for all a and b in M, a . b in M
(associativity) for all a, b and c in M, (a . b) . c = a . (b . c)
(identity) there is an e in M such that for all a, a . e = e . a = a

If V is a subset of M, then V* is defined as the smallest superset of V that contains ε (the empty string) and is closed under the operation. V* is then itself a monoid, and is called the monoid generated by V. This is a generalization of the Kleene star discussed above since the set of all strings over some set of symbols forms a monoid (with string concatenation as binary operation).

The Kleene star is named after Stephen Kleene (1909-1994) who introduced it when describing certain automata (see regular expression).