Re: [patch] gsub with backslash characters in replacement string

On 4/26/07, Adam Bozanich <[e-mail]> wrote:
>
> On 4/26/07, Yukihiro Matsumoto <[e-mail]> wrote:
> >
> > Hi,
> >
> > In message "Re: [patch] gsub with backslash characters in replacement
> > string"
> >     on Thu, 26 Apr 2007 17:09:42 +0900, "Adam Bozanich" <[e-mail] >
> > writes:
> >
> > |Hello, spotted this one the other day:
> > |
> > |irb(main):001:0> '\\'.gsub('\\','\\\\')
> > |=> "\\"
> > |irb(main):002:0> '\\'.gsub('\\','\\\\\\')
> > |=> "\\\\"
> > |
> > |The following patch seems to fix things on my system
> >
> > It was intentional to escape backslashes in replacement string.
> > If it were a bug, how do we escape backslashes?  Any idea?
> >
> >                                                         matz.
> >
> >
> Not sure that I'm understanding you here.
>
> let's try again.
>
> test.rb:
>
> 1.upto(6) do |x|
>     print "a".gsub("a","\\"*x).size , " "
> end
>
> print "a".gsub("a","\n")
>
> before patch output is:
> 1 1 2 2 3 3
>
> after patch output is:
> 1 2 3 4 5 6
>
> Note that the newline works in both cases.
>
> -Adam
>

It has something to do with replacing groups in regular expressions. For
example, you can do:
>> "abc".gsub(/(.)/, "\\1,")
=> "a,b,c,"

So, to escape literal backslashes in replacements, we need four backslashes:
\\\\ # actual code
\\ # actual string
\ # actual replacement
Now if we have an odd number of actual backslash characters, then the last
one will be treated as a literal:
\\\\\\ # actual code
\\\ # actual string
\\ # actual replacement

What we can do is to not double-escape the backslashes if the first argument
is non-regex. Though that maybe a little confusing especially if you expect
your first argument to be both string and regex.


-- 
"Life is unfair... but beautiful."
Scarlette Krimson