s#^(\S*(?<=\W)re(?=\W)\S*\s*)+##i

##</code><code>##

    my $depth= 1;
   while(  $title =~ s#^(\S*(?<=\W)re(?=\W)\S*)\s*##  ) {
        my $re= $1;
        $depth += ( $re =~ /(\d+)/ ) ? $1 : 1;
   }
   $title= "Re$depth: $title";