my $foo = "x.....xxx.x.....x......xxx...xx...x...xxx";

my @array =
$foo =~ 
/(                              # capture in $1
[x]                             # match an opening x
        (?:[^x]*?[x][^x]*?)     # match an x surrounded by some non-x
        {2}                     # match n-2 times
[x]                             # match a closing x
)/gx;
#(?{push @array, $1})            # keep the match
#$                               # match the end of the string to force back-tracking

print "$_\n" foreach (sort {length $a <=> length $b} @array);

__END__
x.....xxx
x...xx...x
x.....x......xx