The code is probably very inefficient and ugly, but it works.
Run ghc re_tools.hs && ./re_tools | dot -T svg > o.svg && open o.svg.
The regex syntax supports a-zA-Z for the alphabet, concatenation, and
union via | as usual. The following repetition modifiers are supported
as well: ?, +, *, {N} (exactly N), {LO,HI} (LO-HI), {LO,} (LO-),
{,HI} (0-HI).