Simple:

!/neither.*nor/ && /nor/
[download]

Complicated (and surely slower):

/^(?:(?!neither).)*nor/
[download]

Update: Oops, they're not quite equivalent.
The first will match when no "neither" is found before the last "nor".
The second will match when no "neither" is found before the first "nor".

You could probably make the first regex use a non-greedy .*
I think that's the simpler approach rather than negative look behind and what not. The same can be applied to many other programming logic problems..useful sometimes to think about chunks of code logic or regex patterns as sets in finite space.
the hardest line to type correctly is: stty erase ^H

You could probably make the first regex use a non-greedy .*

Wouldn't help.

>perl -E"$_='nor neither nor'; say !/neither.*nor/ && /nor/ ?1:0"
0

>perl -E"$_='nor neither nor'; say !/neither.*?nor/ && /nor/ ?1:0"
0
[download]

Non-greediness is very fragile and frequently misused.

I believe that the following regex should do the job.

No, it disallows following instances of "neither".

$ perl -E'say "a nor b neither" =~ /^(?!.*neither).*nor.*$/mi ?1:0'
0
[download]

Nice catch. Didn't even test that kind of scenario.

Took a few minutes after reading your post to realize why my suggested regex was incorrect. Another example of Perl doing what I told it to do instead of what I wanted it to do. :D

1. The Monastery is NOT a code writing service. Rather, it seeks to be a site to help you learn (cf, "feed a man a fish...." and similar). To that end, "how should I (write some particular code)" is a question that tends to receive disparaging replies... or guides like this on some of the most basic protocols here.
2. If you pose your question -- with code (and data if relevant) -- to seek assistance on a particular issue that has you stumped, you will have learned something in the course of writing that code and exploring the related documentation and you'll usually find that the Monks will be generous with their time and expertise.

Thanks to ikegami (!), aquarium & dasgar for politely pointing me in the right direction. Thank you ww (assuming, that is, it is not some automatically generated reply which is triggered by the pattern /how should i/ig ) for warning me from the undesirable consequences my message may inflict (the worst of all is the re-reading the "how to pose a question" tutorial).

though not a one liner the following is simple to understand and does the trick

$_='nor neither nor'; 
my $cnt = 0;
while ((/(.*?)nor/ig) ) { # =================== coarse match
    printf ("\n");
    printf (STDOUT qq/\$\`:%s\n/,$`);
    printf (STDOUT qq/\$\&:%s\n/,$&);
    printf (STDOUT qq/\$\':%s\n/,$');
    if ($1 !~ /\bneither\b/i){ # =========== finer match
        printf ("match no. %d: ",$cnt++);
        printf ("matched ok\n");
    } 
    else {
        printf ("match no. %d: ",$cnt++);
        printf ("no match\n");
    };
}
[download]

You probably want \b around "nor" too, lest you get caught snoring on the job.