[c-nsp] cisco regex puzzle of the day

[Jared Mauch]

	We've long had some feature requests open against JunOS for
as-path matching.  The challenges faced are they don't treat these
AS numbers as strings, and certainly not in the case of some elements
like a paren (confed) or { for AS_SET.

	In IOS-XR you can much more easily match against the origin-asn
in a policy as well which isn't quite as easy in other routing operating
systems.

	- Jared

On Wed, Mar 11, 2015 at 05:28:06PM +0000, Mack McBride wrote:
> 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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-- 
Jared Mauch  | pgp key available via finger from jared at puck.nether.net
clue++;      | http://puck.nether.net/~jared/  My statements are only mine.