Re: Dynamic regex assertions, capturing groups, and parsers: joy and terror

[demerphq]

I find this quite confusing as well.

Update: No this makes perfect sense. Thanks to dio for straightening me out.

$rx = qr{ (.) (??{ print $1 }) }x;
print "!" if "japhy" =~ $rx;
__END__
japhy

How does $1 end up being 'japhy' with this re? Interestingly, changing it to

$rx = qr{ (.) (??{ print $1; '' }) }x;
print "!" if "japhy" =~ $rx;
__END__
j!

makes things work out properly.

[diotalevi]

The result of (??{ print $1 }) is 1 because print() succeeded in writing to STDOUT. The regex that was then compiled by (??{ ...}) was "1" which then failed. So the (.) advanced over every character and printed them individually. The proper thing to do here would have been (?{ ... }) which will not affect regex matching.

[demerphq]

Doh. Of course. I knew that the print returning 1 failed the match, but i didn't put two and two together to realize that was why all of the chars were printed. And I've used this technique deliberately before too. /gah.

Thanks for the clue-by-four. :-)