What you describe already exists (try posting something in plain text to see). Anything more will require another layer that delays/prevents a post from being submitted so - yes, it would need to be a /very/ good parser. A parser that would be much more work than would be worth the trouble considering SoPW doesn't seem to get more than a total of 25 questions per day.
The handful of weekly "evil markup violators" can easily be handled with a quick private /msg with suggestions to improve their post. If they ignore it (which does happen), ignore the post and move on. ;-)
"...the adversities born of well-placed thoughts should be considered mercies rather than misfortunes." — Don Quixote