From: mike@... (Mike Stok)
Date: 2004-01-22T21:00:10+09:00
Subject: Re: gsub oddity?

In article <buo0rc$j5ef5$1@ID-52924.news.uni-berlin.de>,
Robert Klemme <bob.news@gmx.net> wrote:
>
>"Mark J. Reed" <markjreed@mail.com> schrieb im Newsbeitrag
>news:20040121221951.GD16733@mulan.thereeds.org...
>> On Thu, Jan 22, 2004 at 06:57:22AM +0900, Ralph Mason wrote:
>> > What am I missing here.  I am trying to replace a single quote with
>the
>> > string \'
>>
>> You're missing an extra level of processing applied to the replacement
>> string for gsub in order to let you include parts of what you're
>> replacing.
>
>No, in this case he's missing extra backslashes:
>
>irb(main):002:0> str='a\'o'
>=> "a'o"
>irb(main):003:0> str.gsub(/'/, '\\\\\'' )
>=> "a\\'o"
>irb(main):004:0> puts str.gsub(/'/, '\\\\\'' )
>a\'o
>=> nil
>
>It does look a bit odd, but the explanation is logical: There are two
>levels of escaping involved.  The first level is that needed for the ruby
>parser to get the expected characters into the replacement string.  The
>second level is used in order to be able to use a backslash literally in
>the substitution string.
>
>irb(main):005:0> puts '\\\\\''
>\\'
>=> nil
>
>You need two backslashes in the substitution string to prevent gsub from
>interpreting the backslash as escape char.  That makes four backslashes
>when entering the string.  The fifth backslash escapes the single quote in
>order to be able to include it into the string.

You can make life a little easier by picking a quote other than ' to
get rid of one backslash e.g.

[mike@ratdog mike]$ irb --simple-prompt
>> str = "a'o"
=> "a'o"
>> puts str.gsub(/'/, "\\\\'")
a\'o
=> nil

It can be tiresome to remember how many times a string will be scanned
in a context where \ characters matter, so I prefer using the block form
e.g.

>> puts str.gsub(/(')/) { |c| "\\#{c}" }
a\'o
=> nil

....but I have never measured whether this is significantly more
expensive.

Hal, is this a FAQ?  "How do I replace ' with \' in a string?"  I'm sure
that this has come up a few times...

Hope this helps,

Mike

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