Um, no. Not sure, sure, but that is what I believe to be the case following various discussions regarding the use of qr// and the contrast between it, using /o on a specific m// or s///, and the utility (or otherwise) of using the /o operator in conjuction with qr//o.

What I think (though) that the concensus was that using /o on the qr// operator was a waste of time as qr// effectively already did once-only compilation.

It's quite possible I have misinterreted this (again), and given that it's you that challenging my assumptions, and your likely to reply with a Devel::* dump demostrating that this is not the case, I'll turn the statement into a question and ask....Is this slightly dangerous?

I threw some debug-foo at this code fragment and came up with yes and no. Looking at the code via B::Concise it looks like the regex is compiled each time. Looking at the code view use re 'debug' it looks like it's compiled only once. I suppose that enough digging through perl's guts would provide a real answer but really -- I'd rather just use qr to control when the regex is compiled and not have to worry about what the internals are doing.

I mean, there are times it's interesting and useful to know what's happening but this case is just too opaque to get at.

my @in = qw(nc01123.log
            nc02123.log
            nc03123.log);
my $year = '0';
my $month = '2';
@in = grep { /^nc$year$month.*\.log$/ } @in;
print "@in\n";
__END__

Try zero: The nested sub block was hard to read so this didn't yield t
+oo much.
perl -MO=Concise test.pl

Try one: This makes it look like the regex is compiled anew each time.
perl -MO=Concise,-exec test.pl

Try two: This looks like the regex is compiled only once. Or maybe deb
+ug just <i>dumps</i> it once.
use re 'debug';
[download]

---

Fun Fun Fun in the Fluffy Chair

A rather simpler test method, but in this, (only) "line 1" is printed out 10 times.

#! perl -sw
use strict;

my $count=1;
my $regex = qr/$count/;
my $told = tell DATA;
my $line;
for $count (1 .. 10) {
    $line =~ $regex and print $line while $line = <DATA>;
    seek DATA, $told, 0;
}

__DATA__
line 1
line 2
line 3
line 4
line 5
line 6
line 7
line 8
line 9
[download]

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Examine what is said, not who speaks.