use strict;
use diagnostics;

while( <DATA> )
{   my @list = ();
    if( @list = ( $_ =~ m|^rs://([-\w]+)(?:\.([-\w]+))+|) )
    {
        print join( ' > ', @list ), "\n"; 
    }
}
__DATA__
rs://a
rs://b.c
rs://d.e.f
rs://g.h.i.j.k