Abstract
The expressive capacity of regular expressions without concatenation, but with complementation and a finite set of words as literals is studied. In particular, a characterization of unary concatenation-free languages by the Boolean closure of certain sets of languages is shown. The characterization is then used to derive regular languages that are not concatenation free. Closure properties of the family of concatenation-free languages are derived. Furthermore, the position of the family in the subregular hierarchy is considered and settled for the unary case. In particular, there are concatenation-free languages that do not belong to all of the families in the hierarchy. Moreover, except for comets, all of the families in the subregular hierarchy considered are strictly included in the family of concatenation-free languages.
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