I was surprised then not surprised that the union & intersection REs it comes up with are not particularly concise. For example the two expressions "y.+" and ".+z" have a very simple intersection: "y.*z" (equality verified by the page, assuming I haven't typo'd anything). But the tool gives
yz([^z][^z]*z|z)*|y[^z](zz*[^z]|[^z])*zz*
instead. I think there are reasons it gives the answer it does, and giving a minimal (by RE length in characters or whatever) regular expression is probably a lot harder.
I was surprised then not surprised that the union & intersection REs it comes up with are not particularly concise. For example the two expressions "y.+" and ".+z" have a very simple intersection: "y.*z" (equality verified by the page, assuming I haven't typo'd anything). But the tool gives
instead. I think there are reasons it gives the answer it does, and giving a minimal (by RE length in characters or whatever) regular expression is probably a lot harder.