explanation on a BGP regex..

[nikhil.kulkarni]

Team,

I am a bit new to this regular expressions and to troubleshoot some routing I want to know what the following regex exactly means:

^(65030)?$

Regards,

Nikhil.

[Peter Paluch · Accepted solution]

Hello Nikhil,

Let's break down the regex:

• ^ - this symbol stands for the beginning of a string. It means that the matched string must exactly start with the expression that immediately follows the ^ symbol.
• (65030) - this is a constant string, in this case, the AS number. Having it enclosed in parentheses allows subsequent operators to act on the entire string - otherwise, they would act only on the last character preceding them.
• ? - this symbol is a quantifier saying that the preceding character or group of characters may appear at most once, i.e. either not be present or be present exactly once
• $ - this symbol stands for the end of a string. It means that the matched string must exactly end with the expression that immediately precedes the $ symbol.

So the regex ^(65030)?$ stands either for "" or for "65030". In the AS_PATH terms, it means "a prefix either originated in the local AS or originated in the neighboring AS 65030".

Please feel welcome to ask further!

Best regards,

Peter

[nikhil.kulkarni]

Thank You Peter. :-)