BGP Regular Expressions Examples

Regular Expressions are used often for BGP route manipulation or filtering. In this lesson 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]+)_51	matches 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!