p6rt
commented
6 years ago From @smls
When backtracking is disabled for an alternation (via `:` modifier or `:ratchet` mode), this obviously means that once the alternation has matched once it may no longer reconsider which branch it wants to select:
say "ab" ~~ / [ab || a ]: b /; # Nil
But does it also mean that once the alternation has matched, the selected branch *itself* may no longer do any backtracking?
I would have thought so, but Rakudo's current behavior is inconsistent:
say "ab" ~~ / [ .+ | x ]: b /; # Nil say "ab" ~~ / [ .+ || x ]: b /; # Nil
say "ab" ~~ / [ x | .+ ]: b /; # Nil say "ab" ~~ / [ x || .+ ]: b /; # ｢ab｣
I.e. in most cases, it commits to what the alternation matched the first time. But when
1) it is an `||` alternation, 2) and its *last* branch matched, 3) and said branch is a backtrackable atom which (unlike the alternation itself) is not affected by `:ratchet`,
then it looks like that backtrackable atom can reconsider what it wants to match even after the regex execution has moved past the alternation.
I suppose another way to phrase the @LARRY design question is this: Does an alternation select a particular match value (like a "function"), or does it select a subregex (like a "macro")?
(PS: I should note that the backtracking behaviors of || and | were even less consistent before my NQP commit 263257a9c¹ [see RT #130117²], which was supposed to bring `||` in line. But I didn't discover this edge case until now, and indeed am not sure how to fix it no matter which way it *should* be fixed.)
[1] https://github.com/perl6/nqp/commit/263257a9c [2] https://rt-archive.perl.org/perl6/Ticket/Display.html?id=130117
This is Rakudo version 2017.09-203-g98fae3d84 built on MoarVM version 2017.09.1-553-ga4fef0bd implementing Perl 6.c.
Migrated from rt.perl.org#132219 (status was 'new')
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