A regular language is said to be star-free if it can be described by a regular expression constructed from the letters of the alphabet, the empty set symbol, all boolean operators – including complementation – and concatenation but no Kleene star.
A regular language is termed star-free if it can be expressed using a regular expression composed of alphabet letters, the null set symbol, all boolean operations (including negation), and concatenation without employing the Kleene star operator.