The FSA Utilities is an open-source software tool, written in Prolog by Gertjan van Noord, for creating, visualizing, and manipulating Finite state machines. It is useful for constructing finite-state machines from regular expressions and performing standard algorithms such as automata determinization, minimization, and intersection; transducer composition; etc.It includes algorithms for weighted and unweighted finite-state acceptors and transducers.It has a Tcl/Tk graphical user interface whichallows the user to view and manually manipulate the shapes of small automata and transducers.There is also a Prolog-based macro language which facilitates the construction of complex regular expressions.