From: Rob Biedenharn <Rob@...>
Date: 2008-02-23T02:48:44+09:00
Subject: Re: gsub("\\", "\\\\") seems unintuitive

On Feb 22, 2008, at 12:27 PM, John Woods wrote:
> The following confusing behavior is noted in the pickaxe book (2nd  
> ed) on page 75:
>
>  # I would expect two backslashes in the result
>  irb> puts "\\".gsub("\\","\\\\")
>  \
>
>  # I would expect four backslashes in the result
>  irb> puts "\\".gsub("\\","\\\\\\\\")
>  \\
>
> I can certainly work around it, but it seems unintuitive. Is there a  
> reason why gsub behaves this way? Just curious...


Notwithstanding the earlier responses...

Since the replacement string is evaluated 'twice', once as a ruby  
string literal and then again by gsub to look for group refrences like  
'\1', you need to provide two levels of escaping for a backslash.

\ is "\\"
so two of them is "\\\\"
and you want gsub to see that so it need to have them escaped:  "\\\\\\ 
\\"

Whew!  Yeah, it's unfortunate, but backslash is doing double-duty  
here: introducing a group reference to the regular expression and  
escaping characters in a string literal (just like "\n", but also  
itself).

-Rob

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