I think I understand everything you are saying, and what I think I wasn't getting is that backtracking through a SKIP immediately terminates the attempt of the regex match at its current position; I wasn't thinking it was a goto, just that somehow I had to do something else to keep the current match attempt from proceeding on other branches.

I don't think I understand yet why this didn't work, though:

/(*SKIP:go)(.)(?(?=\1)\1(*MARK:go)(*FAIL)|)/
[download]

Is it simply that the go mark is looked for when the SKIP is encountered, not when it is backtracked through?