Thanks, now I get the idea and I have to admit that I missed this feature in the past, though I can't recall when exactly.

But all useful use-cases which come to my mind involve strict boundaries, like parsing a grammar and explicitely excluding certain commands.

Like parsing a html but not wanting "a" and "img"-tags while allowing "anchor"

This would mean to use something like

 /<\s*\b(?~a|img)\b(.*)>/

and this should be achievable with

use strict;
use warnings;


undef $/;
$_ = <DATA>;                            # slurp




print "1: $1 - $2\n" while /<\s*(?!a\b|img\b)(\w+)\b(.*?)\s*>/g;

print "2: $1 - $2\n" while /<\s*(?!a|img)(\w+)(.*?)\s*>/g;

my %absent =
  (
   a=>1,
   img =>1
  );


while (/<\s*(\w+)(.*?)\s*>/g) {
   print "3: $1 - $2\n" unless $absent{$1};
}

while (/<\s*(\w+)\b(??{ $absent{$1} ? '^' : '' })(.*?)\s*>/g) {
   print "4: $1 - $2\n";
}




__DATA__
<a href='bla'>
<img href='bla'>
<table style="">
<anchor >
<tr width=100>
[download]

1: table -  style=""
1: anchor - 
1: tr -  width=100
2: table -  style=""
2: tr -  width=100
3: table -  style=""
3: anchor - 
3: tr -  width=100
4: table -  style=""
4: anchor - 
4: tr -  width=100
[download]

please note how important it is to repeat the delimiting \b in case 1, which might justify the use of (?~...) .

Though to be sure I'd need to test the Ruby implementation, and I'm not willing to install yet.

PS: Choosing HTML as data grammar was - as always - unfortunate. You are free to propose something different.