Kleene Star

What Does Kleene Star Mean?

A Kleene star, in the parlance of computer science, is a programming resource that offers outcomes related to the concatenation of a string set. Using the Kleene star, developers and others assess how to filter given results based on input.


A Kleene star is also known as a Kleene operator or Kleene closure.

Techopedia Explains Kleene Star

The Kleene star, which is useful in the building of “automata” or automated systems, is described mathematically as a “unary” operation or a “free monoid” construction. It is described as a possible set of strings that can be created by concatenating elements of a base string. In other words, each individual element of the input string must be present, but additional elements can be repeated to any extent.

A simple alphabetical example can show how the Kleene star works. If the input string is composed of the word “cat,” the set of resulting Kleene star strings will include the results “cat” and “ccat” as well as “ccatt,” “cccat” and “ccaattt.”

Engineers and professionals use the Kleene star make a determination on string sets, for example, to match particular inputs in automation, and work on big data sets coming into an IT system.


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Margaret Rouse
Technology Expert

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.