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.

Oh but that's not at all the same. It's quite obvious (to me anyway) that $count is being compiled each time. The only reason I brought this up was I didn't know whether the invariant regex in a block attacked to a grep was going to be recompiled each time or not. There's an opportunity for perl to notice that the regex only needs to compiled once - the question here is on whether it takes it or not.

Update: Oops! I thought you said $line =~ $count where you actually said $line =~ $regex. Anyhow I still think you've misunderstood which question I was asking.

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Fun Fun Fun in the Fluffy Chair