Summary
Add documentation for regular expressions for as-path-lists.
Use case
Writing regular expression is very hard, if the syntax is unspecified. https://docs.vyos.io/en/latest/configuration/policy/as-path-list.html gives a trivial example. I assume, that this gets sent as is to FRR, however as far as I can tell FRR also has not documented what they are doing here. The only thing that I found is this example here: https://docs.frrouting.org/en/latest/bgp.html#bogon-asn-filter-policy-configuration-example
Additional information
The NLNOG route filtering guide gives examples for matching ranges of ASNs, however these examples show a different syntax that what the "documentation" at FRR shows. https://bgpfilterguide.nlnog.net/guides/bogon_asns/#frr-vtysh
There are also examples for VyOS there, which are basically the FRR config wrapped in VyOS commands with the same expressions.