From: Janus Bor <janus@...>
Date: 2008-08-17T01:46:45+09:00
Subject: Re: Alternative for look behind

Thanks to both of you! I like your solutions. I'm a fan of one liners...

I ended up using this one, as it has the best readability for a novice 
programmer like me imho:

Robert Klemme wrote:
> irb(main):011:0> sequence.sub(/^-*/) {|m| "X" * m.length}
> => "XXXXXXXabcde-gh----"


Robert Klemme wrote:
> It does - in 1.9.  But:
> 
> irb(main):001:0> sequence = '-------abcde-gh----'
> => "-------abcde-gh----"
> irb(main):002:0> sequence.gsub(/(?=^-*)-/, "X")
> => "X------abcde-gh----"

Thanks for the info. Just downloaded 1.9 to check it out. But I still 
don't comprehend why look behind is not working like I expected. If I 
write the equivalent replacing characters at the end of the string using 
look ahead it works just fine:

irb(main):001:0> sequence = '-------abcde-gh----'
=> "-------abcde-gh----"
irb(main):002:0> sequence.gsub(/-(?=-*$)/, "X")
=> "-------abcde-ghXXXX"

Robert Klemme wrote:
> I am not sure lookbehind is the proper means here.  After all you want
> to replace a sequence of dashes *before* a particular sequence
> (according to your description above).  So you would rather use
> lookforward, wouldn't you?
> 
> irb(main):003:0> sequence.gsub(/-(?=-*abcde)/, "X")
> => "XXXXXXXabcde-gh----"
> 
> Granted, it does not anchor.

The problem with look ahead/forward is, that I have to make sure the 
sequence starts with "-". Otherwise something like this could happen:

irb(main):003:0> sequence = 'abcde-gh----'
=> "abcde-gh----"
irb(main):004:0> sequence.gsub(/-(?=-*[abcdefgh])/, "X")
=> "abcdeXgh----"


Kind regards,
 Janus
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