Unfortunately we don't currently have an equivalent to the POSIX regexp that would allow one to specify the minimum and maximum number of occurence for a given string as follow:
^12345 ([0-9]_){1,4}$
Given the available regexp set, your as-path list is probably the best I could come up with.
Hope this helps,
Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
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