From: "Ara.T.Howard" <ahoward@...>
Date: 2004-05-14T07:03:58+09:00
Subject: Re: nasty regexp problem

On 12 May 2004, Francis Hwang wrote:

> Hi all,
> 
> I discovered a strange bug in Lafcadio which I'm trying to work out,
> but the regexp is getting really nasty. Basically, to commit text
> values to the database I need to double any apostrophe, except those
> that are preceded by a backslash. This has to account for apostrophes
> at the beginning of the string, and multiline strings where the
> apostrophe is at the start of a new line. Examples would be:
> 
> "I can't drive 55" -> "I can''t drive 55"
> "Don\'t escape here" -> "Don\'t escape here"
> "'" -> "''"
> "line 1\n' line 2" -> "line 1\n'' line 2"
> 
> So here's the regexp that does it:
> 
> value = value.gsub( /(^|[^\\\n])'/ ) { $& + "'" }
> 
> which works fine, except I just discovered that it fails in one odd
> case:
> 
> "'''" -> "'''''"
> 
> (That is, a single line of three apostrophes should become six
> apostrophes, but instead it becomes five.)
> 
> Any idea why this is doing it? I suspect I'm being too clever by
> including the beginning of line in a grouping, and maybe that affects
> how the regexp is processing the string?
> 
> F.

a "'" is escaped iff it is preceded with an odd number of "\" :


~ > cat a.rb
strings = <<-'txt'
  I can't drive 55
  Don\'t escape here
  line 1\n' line 2
  1 '
  2 ''
  3 '''
  4 ''''
  these are escaped \' \\\' \\\\\'
  these are not escaped \\' \\\\' \\\\\\'
txt


pat = %r/([\\]*)'/o

strings.each do |string|
  string.strip!
  escaped = 
    string.gsub(pat) do
      if $1.size % 2 == 0
        "#{ $1 }''"
      else
        $&
      end
    end
  puts "<#{ string }> -> <#{ escaped }>"
end


~ > ruby a.rb  
<I can't drive 55> -> <I can''t drive 55>
<Don\'t escape here> -> <Don\'t escape here>
<line 1\n' line 2> -> <line 1\n'' line 2>
<1 '> -> <1 ''>
<2 ''> -> <2 ''''>
<3 '''> -> <3 ''''''>
<4 ''''> -> <4 ''''''''>
<these are escaped \' \\\' \\\\\'> -> <these are escaped \' \\\' \\\\\'>
<these are not escaped \\' \\\\' \\\\\\'> -> <these are not escaped \\'' \\\\'' \\\\\\''>


i have done this with a simple gsub - but's it's __really__ ugly... i think you
may be able to find it on the list if you look...  in the end it's easier to
use a block and determine if you are dealing with and even or odd number of
'\'s

-a
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