I'd suggest Super Search. Which will basically show that AnomalousMonk is the local expert on the subject :P. After a quick browse, I have extracted two bits of interresting (to me) information. First (*SKIP) and (*FAIL) can be used for something that quacks like variable length look behind assertions, as discussed here. Second, (*FAIL) is identical to (?!), and one is actually compiled to the other.
I find this interesting because I have actually never used any of the control verbs - I remember having a vague understanding of what they do, and how to use them, but not why I would need them - while I have used look-around assertions (mostly look ahead) on several occasions and have a more intuitive understanding of them.
On the "variable length look behind assertion" case though, a far simpler solution has been made available with \K. This may be one reason why (*SKIP) and (*FAIL) aren't very common, but might also be another gateway to understanding what they do