use strict;
use warnings;

my $string = qq[FGTXYZGTFABCGHABCFGTXYZADXYZGTYABC];
#or try this my $string = qq[FGTXYZABCABC];
$string =~ m/[XYZ]{3}([A-Za-z]{0,21}?)[ABC]{3}?(?{print "matched at [<$&>] $-[0]\n" })(?!)/x;