That's a really bad proofreading regex, though, because it will be wrong more than a third of the time--which is much, much higher than the expected real spelli

That's a really bad proofreading regex, though, because it will be wrong more than a third of the time--which is much, much higher than the expected real spelling error rate. The "i before e except after c" rule only has a high success rate when you add the caveat that it only applies to words where the ie or ei makes the sound [i:] (like in "believe", for example), and that caveat isn't expressible via regex.