#!/usr/bin/perl

# https://perlmonks.org/?node_id=11101201

use strict;
use warnings;

#
# Negative lookbehind , huh ??
#
while (<DATA>) {

    if (/^(?!TAGS)(.*?)(?=TAG2)/xgi ) {
        print "Non-Negative lookbehind: ";
        print $1, "\n";
    }
}
#
__DATA__
TAG1 text one TAG2
TAG1 text two TAG2
TAG1 text three  TAG2
TAGS text four   TAG2
TAG1 text five TAGT
TAG1 text six TAG2
[download]

This fails because your lookbehind isn't anchored. You can either anchor it where you want or if it's really always at the start, replace the lookbehind with a lookahead like so:

while (<DATA>) {

    if (/^(?!TAGS)(.*?)(?=TAG2)/xgi ) {
        print "Negative lookahead: ";
        print $1, "\n";
    }
}
#
__DATA__
TAG1 text one TAG2
TAG1 text two TAG2
TAG1 text three  TAG2
TAGS text four   TAG2
TAG1 text five TAGT
TAG1 text six TAG2
[download]

Hello cmic,

Did you notice that for the line beginning with "TAGS", the capture includes that word? Even though it is non-greedy, (.*?) starts looking at the beginning of the string and eventually settles on the capture string "TAGS text four   ", which satisfies both the lookahead (because it is followed by "TAG2") AND the negative lookbehind — because the string (which includes "TAGS") is not preceeded by "TAGS" !

In general, you can’t combine a negative lookbehind assertion with a match-any-character(s) capture. Do this instead:

use strict;
use warnings;

while (<DATA>)
{
    print "$1\n" if !/^TAGS/ && /(.*?)(?=TAG2)/;
}
[download]

Update: Actually, in this case you don’t need a lookahead assertion either. This does just as well:

    print "$1\n" if !/^TAGS/ && /(.*?)TAG2/;
[download]

Hope that helps,

OK. I get it. But I was just trying to use the "negative lookbehind". I already used the solution with positive lookahead or your solution (which is more elegant, for sure). THX for your help