A very good point that is ideed the ultimate answer to my question by
guiding to Special Backtracking Control Verbs.
It states that (*FAIL) can be used to force the engine into backtracking
and that this is equivalent to (?!).
So version 2 and yours are basically the same and both are guaranteed to work.
The trickery from version 3 is not needed.
So in the end it is "use" and not "abuse".
Greetings, -jo
$gryYup$d0ylprbpriprrYpkJl2xyl~rzg??P~5lp2hyl0p$
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I'd call it "abuse". My bet is this pattern of application is well-known and tolerated for the sake of critical mass of existing "cool examples of (ab)using re-engine", and therefore safe to use in the future :). Stand-alone (*F) is guaranteed to fail, there's no need to "force to backtrack" while staying in the same branch; and as there are no other branches in your example, the whole matching must have been optimized away. On the other hand, something like (?(?{CODE})(*F)), with CODE result depending on sub-matches so far, is legitimate use and another matter entirely, but not the case here.
The impression is, aforementioned tolerance goes as far as injection of (*F) makes (but not always) engine fail to fail early, which is funny.
my $match = qr[([ab]+)([ab]+)];
my $str = 'aba';
$str =~ /^ $match $ (?{ print "1: $1-$2\n" }) a /x;
$str =~ /^ $match $ (?{ print "2: $1-$2\n" }) b /x;
$str =~ /^ $match $ (?{ print "3: $1-$2\n" }) (*F) b /x;
$str =~ /^ $match $ (?{ print "4: $1-$2\n" }) (*F) .. /x;
__END__
1: ab-a
1: a-ba
3: ab-a
3: a-ba
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(?{CODE})(?!)
(?{CODE})(*F)
are documented as equivalently forcing the engine to backtrack and are
just what I was looking for.
I don't call this "abuse", but YMMV.
The example with a character class was just historical and the one
with (??{CODE}) was a result of my own ignorance.
Greetings, -jo
$gryYup$d0ylprbpriprrYpkJl2xyl~rzg??P~5lp2hyl0p$
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