From: Robert Klemme <shortcutter@...>
Date: 2008-08-16T18:37:00+09:00
Subject: Re: [Regex] Alternative for look behind

On 16.08.2008 03:56, Janus Bor wrote:
> I need to replace all '-' characters at the beginning of a sequence
> (string) with 'X's (until a character appears that is not '-').
> 
> E.g.
> 
> -------abcde-gh----
> 
> should be changed into
> 
> XXXXXXXabcde-gh----
> 
> If Ruby would support look behind in Regular Expressions, I could
> probably do something like this:
> 
> sequence.gsub(/(?=^-*)-/, "X")
> 
> But unfortunately Ruby does not support look behind.

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----"
irb(main):003:0>

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.

Here's another solution

irb(main):008:0> s  = sequence.dup
=> "-------abcde-gh----"
irb(main):009:0> while s.sub! /^(X*)-/, '\\1X'; end
=> nil
irb(main):010:0> s
=> "XXXXXXXabcde-gh----"

But I'd rather use Pena's solution or another block form

irb(main):011:0> sequence.sub(/^-*/) {|m| "X" * m.length}
=> "XXXXXXXabcde-gh----"
irb(main):012:0> sequence.sub(/^-*/) {|m| m.tr '-','X'}
=> "XXXXXXXabcde-gh----"

Kind regards

	robert