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List:       ruby-talk
Subject:    Regex Question
From:       seijin () gmail ! com
Date:       2007-07-24 16:50:01
Message-ID: 1185295585.795087.31500 () i13g2000prf ! googlegroups ! com
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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?

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!


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