[c-nsp] cisco regex puzzle of the day

[Mack McBride]

There is no back tracking in the junos regex nor would backtracking really help.
Doing this is complicated on cisco due to the lack of negating a full as.

However loop avoidance should prevent 64500 from occurring twice with an intervening AS.
If you have turned off loop avoidance with allowas-in then you have a lot
More complexity to worry about.

I haven't tested this but it should work:

(65400_)+([1-57-9][0-9]*_|6[01-35-9][0-9]*_|64[01-46-9][0-9]*_|645[1-9][0-9]*_|6450[1-9][0-9]*_|64500[0-9]+_)+

Mack McBride | Network Architect | ViaWest, Inc.
O: 720.891.2502 | mack.mcbride at viawest.com | www.viawest.com | LinkedIn | Twitter | YouTube



-----Original Message-----
From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Saku Ytti
Sent: Wednesday, March 11, 2015 10:38 AM
To: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] cisco regex puzzle of the day

On (2015-03-10 20:29 +0100), Job Snijders wrote:

>     "^64500+ [^64500]"
>
> This junos beauty will match for example: "64500 64500 123 123 444",
> but not "64500 64500" or "64500".
>
> Can any of you come up with a single line regex that works on IOS or
> XR
> (ios-regex) to mimick the above described behaviour?

Follow-up question. Is there use-case for regular expression backtracking in AS_PATH?
It would be simpler to implement without backtracking and it would fix this specific use-case, as simple '(64500_)+.+' would work. But perhaps it's still stupid idea, perhaps it'll break lot of really common use-cases.

--
  ++ytti
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