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Dear Monks,

If I use the construct (?(condition)yes-pattern) in a regex, and the condition is the execute-perl-code construct (?{1}), i.e. the condition is always true, the output is as expected:

my $str = 'bxAybz'; while ( $str =~ /(?(?{1})(b[xyz]))/g ) { say 'yes'; say $1; } --output:-- yes bx yes bz

But when I use the condition (?{pos() % 2 == 0}), I expect the same output, but I don't get it:

my $str = 'bxAybz'; while ( $str =~ /(?(?{pos() % 2 == 0})(b[xyz]))/g ) { say 'yes'; say $1; } --output:-- yes bx yes Use of uninitialized value $1 in say at 2.pl line 9. yes bz

Three matches?

Also, I notice the x modifier doesn't work with a conditional regex:

my $str = 'bxAybz'; while ( $str =~ / (? (?{1}) (b[xyz]) ) /gx ) { say 'yes'; say $1; } --output:-- Sequence (? ...) not recognized in regex; marked by <-- HERE in m/ (? <-- HERE (?{1}) (b[xyz]) ) / at 2.pl line 12.
That feels like a bug.

In reply to Conditional regex by 7stud

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    [Corion]: hippo: If you have a Unicode-wise Perl then likely some zero-width characters in $x would work. Maybe $x = "\x{200b}" works.
    [Corion]: Hmm - no, that outputs 1 for me on 5.14 - perl -wle "my $x = qq(\x{200b}); warn $x; warn length $x"
    [hippo]: Smart - I'll give that a go. Thanks.
    [hippo]: Ah
    [Corion]: But maybe there is some other Unicode string that will be true but have a zero width
    [hippo]: For explanation, I've seen this construct in someone else's code (no names, no pack drill) and couldn't think of a situation to trigger it.
    [Corion]: You'll have to look somewhere esoteric for that. Maybe some tied variable or special dualvar can also trigger that. But it's certainly not a common occurrence
    [Corion]: And on 5.20, the following also outputs no find:perl -wle 'for my $x ("\x{2000}".."\ x{1fffff}") { if( $x && ! length $x ) { warn qq(<$x>); warn length $x; die } }'
    [Corion]: (this time on Unix)
    [hippo]: Understood. I'll have to go through the code and see if it's doing anything fancy with ties, dual-vars or non-scalars. In the end, it's probably a bug though.

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