From: "David A. Black" <dblack@...>
Date: 2008-03-29T16:05:56+09:00
Subject: Re: Regular Expression help

Hi --

On Sat, 29 Mar 2008, Tony De wrote:

> On to my next learning exercise.  As I parse a file I need to pull an IP
> address out a line.  Now I thought a regular expression would be the
> ticket, but it's giving me a problem.  The follow line is an example
> string I need to pull one of two IP address out of:  (they are not
> always formed the same)
>
> Received: from mmds-111-19-22-30.twm.ca.internet.net (HELO
> ?192.168.1.2?) (222.222.222.22)
>
> I need that last IP address.  Now the problem is that I can't always
> count on it being enclosed in paren's.  Although I can expect the right
> paren to always be there.
>
> So here's my regex exp:
> sourceip = line.scan(/\b(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})\b/)
> And as you might expect, it is pulling both IP addresses.  Is there a
> way I can adjust the expression to grab the second IP testing for the
> ")" or is there another method I can use?  Short of dissecting the
> entire string backwards and testing whether I have a number or a char,
> decimal and at most 3 chars from it, etc?

What you want is an IP address, possibly followed by ')' and
definitely coming at the end of the string (give or take a newline
character after it). That can be expressed like this:

   /((\d{1,3}\.){3}\d{1,3})(?=\)?\Z)/

I've got 3 occurences of (\d{1,3}\.), followed by the same thing
without a dot. I've stipulated that this submatch be "looking at"
(i.e., positioned just before) an optional ')' followed by the end of
the string. (\Z gives you end of string, ignoring a possible terminal
newline.)

With your line, it gives you:

irb(main):039:0> line[re]   # re.match(line)[0], or whatever
=> "222.222.222.22"


David

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