Below are some regular expressions that I understand:
ip as-path access-list 1 permit ^$ - permits only locally originated BGP routes
ip as-path access-list 1 permit .* - permit any
ip as-path access-list 1 permit _65535$ - permits networks that originated in 65535
There is a regular expression I do not understand which is being utilized on a network:
ip as-path access-list 1 permit 65535$
What does it mean when the underscore is not present before 65535$
Any responses are welcome.
As a side note, if you want to test these without trying to decode the as-path access-list commands, you can run these at the cli as well:
sh ip bgp regexp 65535$
This will show you what it will match on before doing anything that would affect traffic.
as John said, it's the same in your case.
But using another example:
5535$ matches also networks that originated in AS 65535
_5535$ does not match them.
It does the same thing as your _65535$ expression does. The AS-Path has to end with 65535 (actually originate from).