From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Wed Jan 14 2004 - 00:44:39 GMT-3
What exactly are you trying to match?
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
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> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Tim Fletcher
> Sent: Tuesday, January 13, 2004 8:46 PM
> To: Scott Morris; 'Nathasha Aleyevka'; ccielab@groupstudy.com
> Subject: RE: Regular Expressions
>
> Scott,
>
> Well for starters, I get a syntax error when I try to enter "(_|_.*_)*".
> Aside from that, I still disagree with your logic, but I do have take back
> my statement that the asterisk outside the parenthesis is irrelevant. It
> actually can have an impact that is not what you might expect.
>
> _12(_|_.*_)34_
>
> If I understand your point, you would say that the above would match "12
> 34" and "12 45 34", but not "12 45 67 34". I maintain that it would as
> follows.
>
> "_" - matches the beginning of the string
> "12" - would of course match the 12 in the string
> At this point, we have 2 optional matches within the parenthesis. This
> case, will match the 2nd one.
> "_" - would match the space after the "12"
> ".*" would match "45 67"
> "_" - would match the space before the "34"
> "34" - would match the "34"
> "_" - would match the end of the string
>
> The problem with the asterisk outside the parenthesis (aside from the
> syntax error) is that it means match 0 or more of the preceding character,
> or in this case group of characters. So lets look at several ways this
> could work. 1st, we could have 0 occurrences of what's in the parenthesis,
> which would match "1234", which is probably not what we had in mind.
>
> 2nd, it could match 1 occurrence of what's in the parenthesis, which as
> shown above, will match any AS path that went through AS 34 and AS 12 (in
> that order).
>
> For the 3rd case, lets look at what we get if we try to match 2
> occurrences of what's in the parenthesis. Since we have 2 options within
> the parenthesis, this will give 4 possible regular expression combinations
> (the parenthesis are just to make it more readable).
>
> (_)(_)
> (_)(_.*_)
> (_.*_)(_)
> (_.*_)(_.*_)
>
> The problem with these regular expressions is that they all require 2
> characters that math the "_" wildcard in a row. Offhand I can't think of
> any AS path strings that would meet that criteria.
>
> -Tim Fletcher
>
> At 11:56 PM 1/12/04, Scott Morris wrote:
> >Correct. But the "." is part of the string in between spaces "_"... So
> >that makes things a little different. The "*" outside the parentheses
> would
> >be for multiples of that match. Otherwise the string would match only
> one
> >single AS (or just one space) between those two numbers.
> >
> >You have to remember that the match will come as the ENTIRE string that
> you
> >are set up to match. If you don't place wildcards like that "*" outside
> the
> >paren, then you are limiting the overall string size and won't get as
> many
> >matches.
> >
> >That MAY be your intent, but the way I was reading the original post,
> that
> >wasn't the intent. Hence the correction.
> >
> >Hope that helps.
> >
> >Scott
> >
> >
> >-----Original Message-----
> >From: Tim Fletcher [mailto:groupstudy@fletchmail.net]
> >Sent: Monday, January 12, 2004 2:05 PM
> >To: Scott Morris; 'Nathasha Aleyevka'; ccielab@groupstudy.com
> >Subject: RE: Regular Expressions
> >
> >At 05:05 PM 1/10/2004 -0500, Scott Morris wrote:
> ><snip>
> >>If you had the last string as ...75 (since 5 characters is the max set
> >>anyway), then that would be great for anything ending in 75. But the
> >>"(_|_.*_)" refers to the single space or anything within that one AS
> >>set (between spaces). If you wanted to have it pass through multiple
> >>(as many as necessary) systems, you woul duse the "(_|_.*_)*" instead.
> >>That * on the outside of the parentheses treats the entire thing as a "0
> or
> >more of"
> >>match.
> >
> >Scott,
> >
> >I would have to disagree with you on this. Remember that "." matches any
> >character, including spaces, and that wildcards are greedy. So "_.*_"
> will
> >match as many characters, including spaces that it can. So the "*"
> outside
> >the parentheses wouldn't really make any difference.
> >
> >-Tim Fletcher
> >
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