use strict;
use warnings;

while (<DATA>) {
    print "matched $_" if /^[^cat)(?:]/;
}

__DATA__
cat dog
dog cat
aardvark ant
catnip dogbone
frog cat dog
mouse dog cat
cat dog elephant
can of worms

##</code><code>##

matched dog cat
matched frog cat dog
matched mouse dog cat

##</code><code>##

use strict;
use warnings;

while (<DATA>) {
    print "matched $_" if /^(?!cat\b).*(?=\bdog$)/;
}

__DATA__
cat dog
dog cat
catnip dogbone
aardvark ant
frog cat dog
can of worms

##</code><code>##

matched frog cat dog