Moodle Community already have Josephs Rezeau question type; this type was developed out of frustration by work with it and is sowewhat contrary:
it doesn't have any HINT functionality, just regular expression matching check, but it also doesn't have any issues associated with it: it's supports full syntax of php perl-compatible regular expressions (any complexity and with any number of possible matches).

It's also somewhat stable - it is used in Volgograd State Technical University for almost a year, on several courses with several hundreds of quite complex regular expressions.