From: Brian Candler <b.candler@...>
Date: 2010-09-26T07:08:13+09:00
Subject: Re: bug in gsub(?)

Tiziano Merzi wrote:
> I have found this bug(?) in gsub

http://www.catb.org/~esr/faqs/smart-questions.html#id382249

> puts "\\:{}=#~".gsub(/([\\\:\~\=\#\{\}])/, '\\ \1')
>  =>  \ \\ :\ {\ }\ =\ #\ ~   OK
> 
> but
> 
> puts "\\:{}=#~".gsub(/([\\\:\~\=\#\{\}])/, '\\\1')
>  => \1\1\1\1\1\1\1
> 
> Any idea?

puts "a".gsub(/a/, '\\\\')  # i.e. two backslashes
=> \

That is, in a replacement string, if you backslash-escape a backslash 
you get a single backslash. That allows you to have literally \1 if 
that's what you need.

So a literal backslash is \\, and the first capture is \1

So what you want is \\\1, to get a backslash followed by the first 
capture. However, that is represented in a string literal as '\\\\\\1' 
(which generates a 4 character string) because a string literal also has 
backslash escaping.

>> '\\\\\\1'.size
=> 4
>> puts "\\:{}=#~".gsub(/([\\\:\~\=\#\{\}])/, '\\\\\\1')
\\\:\{\}\=\#\~
=> nil

Take a suggestion from me: save your sanity and use the block form 
instead :-)

>> puts "\\:{}=#~".gsub(/([\\\:\~\=\#\{\}])/) { "\\#{$1}" }
\\\:\{\}\=\#\~
=> nil
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