Here is the source to the python meat of this project: http://pastie.org/1180230

The general idea is that it is a command line tool that reads a regex as input (along with a number of examples to generate). It parses the regex into an abstract syntax tree, then hands it over to an interpreter to "execute" the expression/program several times.

The 'simpleparse' python library does a nice job of easing the regex-to-tree mapping.