Explanation of BGP Regular Expressions with Examples

Indication of the Meta Characters

^ - Start of Line

$ - End of Line

| - Logical Or

_ - Any Delimeter

? - Zero occasions of the Preceding character

* - Zero Or Mode occasions of the Preceding character

+ - One Or More occasions of the Preceding character

(x) - Combine the encased String as a solitary element

[x] - Wildcard where any position can match the position in As-Path

. - Any Character

Cases

_65505$ - Prefixes that End with the As 65505, significance - they were begun by that As

_65505_ - Prefixes that navigated the As 65505

^$  - Locally Originated Prefixes (Start and End of the line)

.*  - Any prefix (zero or more occasions of Any character)

^[0-9]+$ - All the prefixes from Directly Connected Ass (significance - they have just 1 As in the As Path)

When Creating The As-Path Acl

Provided that you need to Stop utilizing the recursive calculation as a part of request to have the capacity to control more intricate  customary representations

 (config-router)#bgp regexp deterministic

Presently you can really Display the prefixes that match your condition in the As-Path before characterizing the As-Path Acl

 #show ip bgp regexp Regular_expression

*there is a Trick here, you have to include a Memory area you need to briefly put thhe comes about, so in place of the statement ^300$ you might need to sort:

 #show ip bgp regexp (^300$)(_\1)*$

You can additionally show the Filter List before applying it to the neighbor:

 #show ip bgp channel record 1