From: Robert Klemme <shortcutter@...>
Date: 2008-05-18T20:50:06+09:00
Subject: Re: sub/gsub bug?

On 18.05.2008 13:38, Robert Klemme wrote:
> On 18.05.2008 06:33, Phlip wrote:
>> Matt wrote:
>>
>>> Is this a bug?
>>>
>>> irb(main):032:0> "matt's test".sub(/'/, "\\'")
>>> => "matts tests test"
>>>
>>> It's trying to replace a single quote with an escaped quote.
>>
>> To understand the bug, try this:
> 
> There is no bug.
> 
>> irb(main):009:0> "matt's test".sub(/(')/, '\1')
>> => "matt's test"
>>
>> The 'single ticks' interpret \1 as a literal \ and a 1, not an escape. 
>> Then .sub sees the \1, and replaces it with the 1nth (grouping) from 
>> the Regexp - which is a literal tick '.
>>
>> Now try it with "double quotes":
>>
>> irb(main):005:0> "matt's test".sub(/(')/, "\\1")
>> => "matt's test"
>>
>> The " forces us to escape the \ as \\ to get a literal \.
> 
> And a literal \ in a replacement string is a meta character.  To get a 
> literal in a replacement, you need \\ in the string and consequently 
> \\\\ in a double quoted string.
> 
>> So your \\' indeed goes in as a literal \'. But the group replacer 
>> still sees a \, and snarfs it, expecting a number after it. I would 
>> call that a bug (essentially because I might personally be capable of 
>> correctly parsing a \1!)
> 
> No, this is not a bug and it is not the "group replacer".  The 
> replacement string is parsed by the regexp engine and since the 
> backslash does not escape a character that has meta capabilities (such 
> as & and 1..9) it is silently discarded.
                        ^^^^^^^^^^^^^^^^^^

This was wrong of course as Matt demonstrated

irb(main):019:0> "abc".gsub(/b/, "<\\'>")
=> "a<c>c"

But the backslash is just retained if it does not appear with a meta 
capable character:

irb(main):020:0> "abc".gsub(/b/, "<\\a>")
=> "a<\\a>c"
irb(main):021:0> puts "abc".gsub(/b/, "<\\a>")
a<\a>c
=> nil

Sorry, for the added confusion.

Kind regards

	robert