Question about public identifier

Hi,
I'm working with XML and I have a question about public identifier.
>From the web, it mentioned that formal publicid has to be in certain
format like:
        prefix//owner-identifier//text-class
text-description//language//display-version.
However, W3C does not mention it and implied it could be in any format
what the allowed PubidChar.
I'm wondering if I write a Xml file with
<!DOCTYPE TypeLib PUBLIC "$XILINX/data/xml/Ldm/Ldm_TypeLib.dtd"

"http://web/randd/software/xfndry/rtf/data/xml/Ldm/Ldm_TypeLib.dtd" > ,
in which $XILINX will be resolved by my own EntityResolver, will it be
OK?
Will <!DOCTYPE TypeLib SYSTEM "$XILINX/data/xml/Ldm/Ldm_TypeLib.dtd"> be
acceptable? The concern is that SYSTEM id has to be a URI, while
"$XILINX/data/xml/Ldm/Ldm_TypeLib.dtd" could only be resolved my parse
with my own EntityResolver.

Could you help me to solve the confusion?
Thanks a lot in advance!

Lihong Pei