I'm curious to see what ends up in "21: Extensions To Avoid".

By the way, from a quick glance, I believe there is no chapter devoted to discuss how the regexp engines work (which is ok) but will there be a section on "when things explode in your face due to exponential behaviour"?

I remember when I first noticed a regex I wrote had a bewhaviour like that and it was kind of enlightening, because earlier I had always assumed all the talk about this things was rather academic :)