From: Robert Klemme <shortcutter@...>
Date: 2010-11-22T22:53:02+09:00
Subject: Re: gsub and backslashes

On Mon, Nov 22, 2010 at 1:28 PM, Ammar Ali <ammarabuali@gmail.com> wrote:
> On Mon, Nov 22, 2010 at 10:38 AM, Robert Klemme
> <shortcutter@googlemail.com> wrote:
>> On Mon, Nov 22, 2010 at 12:27 AM, Ammar Ali <ammarabuali@gmail.com> wrote:
>>> On Sun, Nov 21, 2010 at 11:02 PM, Brian Candler <b.candler@pobox.com> wrote:
>>>> Ralph Shnelvar wrote in post #962847:
>>>>>   "\\1\\2\\3".gsub(/\\/,"\\\\")
>>>>
>>>> Here you are replacing one backslash with one backslash.
>>>>
>>>> The trouble is, in the *replacement* string, '\1' has a special meaning
>>>> (insert the value of the first capture). Because of this, a literal
>>>> backslash is backslash-backslash.
>>>
>>> That's a keen observation, but the fact that they happen to be
>>> back-references doesn't seem to play a part in this situation.
>>>
>>>>> "\\a\\b\\c".gsub(/\\/,"\\\\")
>>> => "\\a\\b\\c"
>>>>> "\\a\\b\\c".gsub(/\\/,"\\\\\\")
>>> => "\\\\a\\\\b\\\\c"
>>
>> The key point to understand IMHO is that a backslash is special in
>> replacement strings.  So, whenever one wants to have a literal
>> backslash in a replacement string one needs to escape it and hence
>> have to backslashes.  Coincidentally a backslash is also special in a
>> string (even in a single quoted string).  So you need two levels of
>> escaping, makes 2 * 2 = 4 backslashes on the screen for one literal
>> replacement backslash.
>
> Actually, 3 backslashes will yield one backslash. The first two result
> in one (escaped), and the third one, escaped by the previous escaped
> backslash ends up being one. My second example showed this, using 6
> backslashes instead of 8. Using 4 backslashes works because the second
> pair yields and escaped backslash, but it is not necessary.

That does not work reliably under all circumstances though:

irb(main):006:0> "abc".gsub /./, "\\\n"
=> "\\\n\\\n\\\n"
irb(main):007:0> puts("abc".gsub /./, "\\\n")
\
\
\
=> nil
irb(main):008:0> "abc".gsub /./, "\\\\n"
=> "\\n\\n\\n"
irb(main):009:0> puts("abc".gsub /./, "\\\\n")
\n\n\n
=> nil

It is safer to use 4 backslashes.  This is the only robust way to do
this even though sometimes you can simply use a single backslash (e.g.
\1 instead of \\1) because string parsing is a bit tolerant under some
circumstances:

irb(main):014:0> '\1'
=> "\\1"
irb(main):015:0> '\\1'
=> "\\1"

but

irb(main):019:0> "\n"
=> "\n"
irb(main):020:0> "\\n"
=> "\\n"
irb(main):021:0> "\1"
=> "\x01"
irb(main):022:0> "\\1"
=> "\\1"


Kind regards

robert

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