Ah, you're using /g... in that case I'd just...

use v5.12;

my $a = join '', map{ ('a'..'j')[rand 10] } 1 .. 1000;

while ($a =~ m[(aa..)]g)
{
    next if pos($a) % 4;
    say "Match at ", pos($a), ": ", $1;
}

[download]

It's possible that (?(cond)yes-expr|no-expr) might be able to do what you want, but I've not had much luck with that.

Update: with the hints about pos % 4 below, I've managed to get (?(cond)yes-expr|no-expr) to work. Not sure how well it goes performance-wise in practice, but:

use v5.12;

my $a = join '', map{ ('a'..'j')[rand 10] } 1 .. 1000;;

while ($a =~ m[(?(?{ pos() % 4 })(*F)|(aa..))]g)
{
    say "Match at ", pos($a), ": ", $1;
}

[download]
package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name

In reply to Re^3: A regex that only matches at offset that are multiples of a given N? by tobyink
in thread A regex that only matches at offset that are multiples of a given N? by BrowserUk

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