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. |
https://networklessons.com/bgp/bgp-regular-expressions-examples/
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