04 Jul

BGP as-path regular expressions

A regular expression is the character pattern that can be matched against an input string. Regular expressions can be built using letters (A through Z, a through z), numbers (0 through 9) and other keyboard characters, such as the exclamation point (!) or a tilde (~). A regular expression can be a single-character pattern or a multiple-character pattern. Certain keyboard characters such as caret (^) and dollar sign ($) have special meaning and allow complex regular expressions to be built. Characters with special meaning can be used as simple keyboard characters by preceding them with a backslash (\). When a Border Gateway Protocol (BGP) update exits an Autonomous System (AS), the AS path attribute of the route gets updated. The AS number of the AS is prepended to an existing list of AS numbers. BGP can be configured to use regular expressions for route filtering based on the AS path attribute.


A range is a sequence of characters contained within left and right square brackets. For example: [abcd]


An atom is a single character, such as the following:

. (Matches any single character)

^ (Matches the beginning of the input string)

$ (Matches the end of the input string)

\ (Matches the character)

– (Matches a comma (,), left brace ({), right brace (}), the beginning of the input string, the end of the input string, or a space.


A piece is an atom followed by one of the following symbols:

* (Matches 0 or more sequences of the atom)

+ (Matches 1 or more sequences of the atom)

? (Matches the atom or the null string)


A branch is a 0 or more concatenated pieces.

Examples of regular expressions follow:

a* (Any occurrence of the letter “a”, including none)

a+ ( At least one occurrence of the letter “a” should be present)

ab?a (This matches “aa” or “aba”)

_100_ (Via AS100)

_100$ (Origin AS100)

^100 .* (Coming from AS100)

^$ (Originated from this AS)

Refer to Using Regular Expressions in BGP for sample configurations on regular expression filtering

To test in live network using public looking glass server:

Additional readings: