This is what I came up with. It loops, but should not be too slow. I hope that it's going to work for you. 
#!perl 

use strict;
use warnings;

my $string = qq[FGTXYZGTFABCGHABCFGTXYZADXYZGTYABC];

my ($pos, $nummatch) = (0, 0);

while ($string =~ /^(.{$pos,}?)[XYZ]{3}([A-Za-z]{0,21}?)[ABC]{3}/) {
   my $tpos = $pos;
   $pos = length($1) + 1;
   $nummatch = length($2);
   while ($string =~ /^(.{$tpos,}?)[XYZ]{3}([A-Za-z]{$nummatch,21}?)[A
+BC]{3}/) {
      print length($1) . ', ' . length($2) . $/;
      $nummatch = length($2) + 1;
   }
}
__OUTPUT__
3, 3
3, 8
20, 8
25, 3

[download]
Explanation: $pos contains the minimal offset, $nummatch the minimum number of characters to match. The outer loop iterates through the starting positions, the inner one through the different matching lengths. As the matching is non-greedy, you will get the matches ordered by starting position first, second by length.

In reply to Re(6): nested pattern matching by CombatSquirrel
in thread nested pattern matching by vinforget

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