Have you always wanted your very own (FST) but you couldn't figure out how to use Lucene's crazy APIs? Then today is your lucky day! I just built a that creates an FST from the input/output strings that you enter. If you just want a finite state automaton (no outputs) then enter only inputs, such as : If all of your outputs are then the FST will use numeric outputs, where you sum up the outputs as you traverse a path to get the final output: Finally, if the outputs are then they are treated as strings, in which case you concatenate as you traverse the path: The red arcs are the ones with the NEXT optimization: these arcs do not store a pointer to a node because their to-node is the very next node in the FST. This is a good optimization: it generally results in . The bolded arcs tell you the next node is final; this is most interesting when a prefix of another input is accepted, such as : Here the "r" arc is bolded, telling you that "star" is accepted. Furthermore, that node following the "r" arc has a final output, telling you the overall output for "star" is "abc". The web app is a simple Python WSGI app; source code is . It invokes a simple Java tool as a subprocess; source code (including generics violations!) is .