/\x26\x67(?!\x26\x67)(.(?!\x26\x67)){6}.{2}/s
####
perl -le 'print for map { /ab(?!ab)(.(?!ab)){2}.{2}/s ? $& : () } @ARGV' abhjabdf abasdasd abdasdadsab ababblah
OUTPUTS:
abasda
abdasd
abblah
##
##
/
\x26\x67 # Starting sequence (let's call it AB)
(?!\x26\x67) # Exclude double starting seqeuence, e.g. ABAB
(?: # non-capture
.(?!\x26\x67) # Any character that's not followed by the sequence
){6} # Need N-2 of these
.{2} # And the last two characters can be anything.
# note we know they're not AB because otherwise the
# (N-2)th item in the previous part wouldn't have matched
/sx # Enable comments, and make . mean everything
##
##
my $marker = '&g';
my $N = 8;
my $n1 = $N-2;
/$marker(?!$marker)(?:.(?!$marker)){$n1}.{2}/s