m/^(?! $a ) .*?  $end/sx

##</code><code>##

m/^(?! .*? $a ) .*? $end/sx

##</code><code>##

$_ = '...</code> ... foo';

if (m{^(?! .*? foo ) .*? </code>}sx){
    print "Match\n";
}
# no match

##</code><code>##

m/^ (?: . (?! $a ) )* $end /sx 

##</code><code>##

$_ = 'foo...</code> ... ';

if (m{^(?: . (?! .*? foo ))* </code>}sx){
    print "Match\n";
}
# match

##</code><code>##

m/^ (?: (?! $a ) . )* $end /sx