From: Rob Biedenharn <Rob@...>
Date: 2007-07-25T02:27:07+09:00
Subject: Re: Regex Question

On Jul 24, 2007, at 12:50 PM, seijin@gmail.com wrote:
> I recently started studying Ruby (1.8.6) and things are going well.
>
> However, something bothered me when I got to the regular expression
> section.  This is my first time deal with regular expressions directly
> so I'm completely new to things.  I understand the idea behind regular
> expressions and most of the usage but two of them stuck out for me.
>
> I'm studying the 2nd edition Pragmatic book and using their
> show_regexp method...
>
> def show_regexp(a, re)
>    if a =~ re
>       "#{$`}<<#{$&}>>#{$'}"
>    else
>       "no match"
>    end
> end
>
> ...hopefully I don't get in trouble posting that method here.  So here
> are my two problems...
>
> First, why does ... show_regexp("banana", /(an)*/) ... not match
> "anan" ?  I thought it was a greedy algorithm that tried to match as
> much as possible?

Well, it would but the * is 'zero or more times' and it can match  
zero "an"s before it ever needs to get past the 'b'.  If you change  
the '*' to a '+' and force 'one or more times', then it has to find  
the first "an" and will then greedily consume the next "an" also.

> Second, how does ... show_regexp("Mississippi", /(\w+)\1/) ... work?
> Why in the world does it match "ississ" rather than returning no
> match?  I think most of my problem with this one is not understanding
> the underlying logic used when doing pattern matching with back
> references.  Does it go through and check "Mississippi", "Mississipp"
> down to "M" and then "i", "is", "iss", etc?  Like trying all possible
> combinations in a lock?  I would be extremely grateful if someone
> would do a short step-by-step of how it matches "ississ".  I
> understand what the "\1" does, I just don't understand how the first
> part even gets to the first "iss".
>
> Thanks a lot!

The '+' is greedy so I think it tries \w+ as Mississippi and finds  
that there's no following copy, backs up to try Mississipp and finds  
that 'i' isn't a copy, and so on back to 'M' before moving past 'M'  
to 'ississippi', 'ississipp', 'ississip', etc. until 'iss' has a  
following copy and a match is declared.  Think of it this way:  Until  
the regexp gets to the end, it doesn't know that the string isn't  
"MississippiMississipp".

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com