#!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}?)[ABC]{3}/) {
      print length($1) . ', ' . length($2) . $/;
      $nummatch = length($2) + 1;
   }
}
__OUTPUT__
3, 3
3, 8
20, 8
25, 3