(*THEN) doesn't work in the simplest test possible

[florian-pe]

This code

"abcde" =~ m{
        \A
        (?{ say "branch 1.1" })
        a
        (b (?{ say "branch 2.1: b" }) | . (?{ say "branch 2.2: ." }))
        (*THEN)
        (*FAIL)
    |
        \A
        (?{ say "branch 1.2" })
        a
}x;

outputs

branch 1.1
branch 2.1: b
branch 2.2: .
branch 1.2

(The line branch 2.2: . shouldn't be printed.)

The issue is that the regex engine backtracks into the inner branch 2 while it shouldn't backtrack at all in between the start of the current alternation (or whole match if the top level isn't an alternation) and the predicate (*THEN) if the control flow in the match attempt went after (*THEN), which it has in this case.

(*THEN) also fails to work properly in the case where the top level isn't an alternation:

"abcde" =~ m{
        \A
        (?{ say "start" })
        a
        (b (?{ say "branch 2.1: b" }) | . (?{ say "branch 2.2: ." }))
        (*THEN)
        (*FAIL)
}x;

which outputs

start
branch 2.1: b
branch 2.2: .

again the line branch 2.2: . should not be printed.

This was already reported in the issue #17069.

In these 2 test cases, replacing (*THEN) by (*PRUNE) does cut the backtracking normally.

[florian-pe]

On a side note, do we need to have github repository privileges to remove the "Needs Triage" label and add new ones?

[jkeenan]

On a side note, do we need to have github repository privileges to remove the "Needs Triage" label and add new ones?

I believe the answer to those two questions is "Yes." "Needs Triage" is, I believe, automatically added to every new Issue. Other labels, e.g., "documentation," may be added if you select one of the other options at https://github.com/Perl/perl5/issues/new/choose.

If you have further questions about this or want to discuss it further, that discussion should not take place inside this or any GH issue. It's a policy issue, and so it should be discussed on the perl5-porters mailing list.