There are the edge cases of foobar:gonkogonk and foobar:ogonkgonk, which I've added to my below test cases. You need to decide if gonkogonk and ogonkgonk should be rejected or accepted. My solution rejects the first but accepts the second case.

use strict;
use Test::More tests => 6;

sub ungonk {
  local $_ = $_[0];
  if (/^(foobar:(?!gonk).{1,5})gonk$/) {
    return $1
  } else {
    return undef
  };
};

is ungonk('foobar:hellogonk'), 'foobar:hello';
is ungonk('foobar:gonk'), undef;
is ungonk('foobar:higonk'), 'foobar:hi';
is ungonk('foobar:helloworldgonk'), undef;
is ungonk('foobar:gonkgonk'), undef;
is ungonk('foobar:gonkogonk'), undef;
is ungonk('foobar:ogonkgonk'), 'ogonk';

[download]

Update: After reading your specification again, you don't want gonk to be found within the first five characters, but it must appear at the end. I think the below program does that, and rejects :ogonkgonk and :gonkogonk.

use strict;
use Test::More tests => 7;

sub ungonk {
  local $_ = $_[0];
  if (/^(foobar:(?:(?!gonk).){1,5})gonk$/) {
    return $1
  } else {
    return undef
  };
};

is ungonk('foobar:hellogonk'), 'foobar:hello';
is ungonk('foobar:gonk'), undef;
is ungonk('foobar:higonk'), 'foobar:hi';
is ungonk('foobar:helloworldgonk'), undef;
is ungonk('foobar:gonkgonk'), undef;
is ungonk('foobar:gonkogonk'), undef;
is ungonk('foobar:ogonkgonk'), undef;

[download]

In reply to Re^3: RegEx - match !foo followed by foo by Corion
in thread RegEx - match !foo followed by foo by Melly

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