AS path access list question

["Todd Caine" <]

Hello.

One of my upstream providers filters solely on AS path
filters.  Generally we nacker by sending them an email which
contains a list of AS path regular expressions.  The AS path
filters start with our local ASN followed by the ASN of a
BGP peer.  Each ASN that becomes a BGP peer with us tells us
which ASNs they will provide transit for.  This information
allows us to make very specific AS path filters for each BGP
peer.  Now on to the problem...

Assume that I am ASX and I provide transit for the AS set
ASX:AS-CUSTOMER.  One of my customers found in the
ASX:AS-CUSTOMER AS set, ASY, provides transit for ASZ and
ASZZ.

The cisco regular expressions that I want to generate for
this customer are:

^(_X)+(_Y)+$
^(_X)+(_Y)+(_Z)+$
^(_X)+(_Y)+(_ZZ)+$

I understand that these 3 above can be consolidated to a
single regexp.  What I don't understand is how I can
generate an AS path filter that allows my customer ASY to
provide transit for specific ASes.

By using RtConfig with the senario above all I seem to be
able to generate is expressions like:

^(_X)+(_Y)+$
^(_X)+(_Z)+$
^(_X)+(_ZZ)+$

which is clearly not what is needed.  Is RtConfig using
AS-SET objects to generate AS-PATH filters?  If so how can
the source AS be determined?

Let me know if any of this makes sense.

Thanks,
Todd