Re: BGP Regular Expression

^  matches the beginning of a line (everything in
life has a start)

[0-9]  includes any number between 0 and 9

+ after [0-9]  means any number between 0 and 9 must be repeated 1 or
more times.

( )  groups multiple expressions together.

_  Underscore will match any Delimiter (including
beginning, end, space, tab, comma)
                In this example it will most likely match a SPACE.

\1  references the previous Parenthesis ([0-9]+)

(_\1)  means that any number 0-9 may repeat x amount of times
preceded by a SPACE, or beginning-of line

(_\1)*  the * means the previous statement can be repeated zero
or more times.

$  in this case: run the loop from the previous statement
until the number sequence is followed by a end-of-line.

The short version :
If used in a as-path access-list, it will match ANY neighboring ASN
while allowing that AS to prepend their ASN x amount of times.

HTH

-- 
<ruhann>
www.routing-bits.com
On Mon, May 24, 2010 at 8:27 AM, Raghav Bhargava
<raghavbhargava12_at_gmail.com> wrote:
> Hi Experts,
>
> Can anyone please explain this Regular Expression:
>
> ^([0-9]+)(_\1)*$
>
>
> --
> Warm Regards
> Raghav
>
>
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-- 
<ruhann>
www.routing-bits.com
Blogs and organic groups at http://www.ccie.net