With the following entry in the catalog file:
{code}
<rewriteSystem systemIdStartString="http://remote.com/dtd"
                   rewritePrefix="file:///share/docbook/docbook/pass"/>
{code}, match for system id "http://remote.com/dtd/book.dtd" returns "file:/share/docbook/docbook/pass//book.dtd". Notice the extra '/' before book.dtd.