BGP Regular Expressions ExamplesRegular Expressions are used often for BGP route manipulation or filtering. In this article we’ll take a look at some useful regular expressions. First let’s take a look at the different characters that we can use:

Characters? repeats the previous character one or zero times.

* repeats the previous character zero or many times.

+ repeats the previous character one or more times.

^ matches the beginning of a string.

$ matches the end of a string.

[] is a range.

_ matches the space between AS numbers or the end of the AS PATH list.

\\ is an escape character. You’ll need this for BGP confederations.

Examples^$ matches an empty AS PATH so it will match all prefixes from the local AS.

^51_ matches prefixes from AS 51 that is directly connected to our AS.

 _51_ matches prefixes that transit AS 51.

 _51$matches prefixes that originated in AS 51, the $ ensures that it’s the beginning of the AS PATH.

 ^([0-9]+)_51matches prefixes from AS 51 where AS 51 is behind one of our directly connected AS’es.

 ^51_([0-9]+) matches prefixes from the clients of directly connected AS 51.

 ^(51_)+([0-9]+)

matches prefixes from the clients of directly connected AS 51, where AS 51 might be doing AS PATH prepending.

 ^51_([0-9]+_)+

matches prefixes from the clients of directly connected AS 51, where the clients might be doing AS PATH prepending.

^\65200\) matches prefixed from confederation peer 65200.

If you need some practice for these, I would suggest to use a BGP looking glass server.

Got some more useful BGP regular expressions? please let me know!