BGP regex question.

[nikhil.kulkarni]

Folks,

I am looking to combine more than one regex into a single expression:

I have regex number 65001, 65002, 65003, 65004 upto 65030 into a single regex.

ip as-path access-list 10 permit _65001$

ip as-path access-list 20 permit _65002$

ip as-path access-list 30 permit _65003$

.

.

.

ip as-path access-list 300 permit _65030$

Is this below regex correct?

ip as-path access-list 10 permit _650[01-30]$

Thanks,

Nik

[cadet alain]

Hi,

I don't think this is possible with one line but i'm not a regexp expert though.

Anyway I think it can be done in 2 lines with:

ip as-path access-list 10 permit _650[0-2][0-9]$

ip as-path access-list 20 permit _65030$

Regards.

Alain

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[Peter Paluch]

Hi Nik and Alain,

Nik, the abbreviated as-path ACL you have suggested is not correct. The expression in brackets is always evaluated as a single character from a defined set. Your expression [01-30] defines a set that contains digits '0' (defined twice), '1', '2', and '3'. It is only a single digit, though. Your abbreviated as-path ACL therefore results into just four AS numbers: 6500, 6501, 6502, and 6503.

Alain, I believe it is possible to create a single-line as-path ACL that will do what Nik requires, however, the resulting regexp will require using the alternation operator. I believe you are correct in your assumption that you cannot write a single regular expression without this operator that will correctly match the range 65000 - 65030. The regexp can be, e.g.:

_650([0-2][0-9]|30)^

It has to be noted, though, that this form strongly reduces the readability and clarity of this expression, and therefore, I would not recommend using it. Over-optimizing things actually degrades them.

Best regards,

Peter

[cadet alain]

Hi Peter,

I'll try it on GNS3 , thanks for the info.

Regards.

Alain

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[Joseph W. Doherty]

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Posting

_650([0-2][1-9]|10|20|30)_

Above is a variation of what Peter suggests, excluding 65000.