Re: understanding (*SKIP:...)

[ikegami]

That (*SKIP:name) precedes (*MARK:name) in your pattern makes no sense, and leads me to think you believe a MARK is akin to a label and a SKIP is akin to a goto. That is not the case at all.

What we're marking and skipping to are positions in the string being matched.

(*MARK:name) bookmarks the current match position under the provided name.
(*SKIP:name) prevents everything before the named bookmarked position from being part of the final match if we backtracking through this.
(*SKIP) prevents everything before the position from being of the final match if we backtracking through this.
(*SKIP) is basically the same as (*MARK:anon)(*SKIP:anon).

Solution:

if ( "aab" =~ /(.)\1(*SKIP)(*FAIL)|.*/ ) {
   say $&;  # b
}

(*SKIP) was matched at position 2. When (*FAIL) caused the matching to backtrack through (*SKIP), everything before position 2 was eliminated from potential matches.

Non-trivial example using (*MARK:...):

If instead you want ab, you could use

if ( "aab" =~ /(.)(*MARK:go)\1(*SKIP:go)(*FAIL)|.*/ ) {
   say $&;  # ab
}

This time, only the text before position 1 was eliminated from potential matches.

[ysth]

Thanks for your help. my actual code using this

[ysth]

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)|)/

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

[ikegami]

Either that, or the mark is scoped so that backtracking through it forgets it. Or both.