Substitute for variable-length look-behind?

[diotalevi]

use Term::ANSIColor ':constants';

# Note that there is not normally a space betweeh the </ and code>. I 
+added that so that Perlmonks.org wouldn't parse the post incorrectly.
$_ = "normal <code> green <!-- yellow [red] normal --> normal </ code>
+ normal";
$GREEN = GREEN;
$YELLOW = YELLOW;
$RED = RED;
$RESET = RESET;
s(<code>(.+?)</ code>)($GREEN$1$RESET)g;
s((?<=\x1b)\[(.+?)\])($RED$1$RESET)g;
s((<!--.+?-->))($YELLOW$1$RESET)g;

The preceding program produces the following string. Overlapping is not detected and each markup element terminates the enclosing element prematurely.

"normal " . GREEN . " green " . YELLOW . "<!-- yellow " . RED . "red" . RESET . " normal -->" . RESET . " normal" . RESET . " normal"

I'd like it to produce this. If perl had variable length look-behind I'd write this as (?<=(?:^|$RESET)[^\x1b]*). Any ideas for how to write this without using perl's experimental regexp features ( (?{ code }), (??{ rx }), and (?(expr)true|false))?

"normal " . GREEN . " green <!-- yellow [red] normal --> normal" . RESET . " normal"