_________XX_X_XX_________ => 2,1,2
__________XXXXX__________ => 5
____XXXX___XXX__XXXXX____ => 4,3,5

##</code><code>##

sub template
{
   my $r = shift;
   $r =~ s/^_+//;
   $r =~ s/_+$//;
   $r =~ s/_+/_/g;
   return join ',', map { length($_) } split(/_/,$r);
}

##</code><code>##

s/(X+)/length($1)/g;