From: Brian Candler <b.candler@...>
Date: 2010-01-08T19:40:59+09:00
Subject: Re: Stopping String Escaping.

Phil Cooper-king wrote:
>> Hopefully it's clear from the above
> yes, thanks you.
> 
>> So try your test case again:
>> (1) Use the DATA.read / __END__ to get the test source in
>> (2) Use 'puts' and not 'dump' to see clearly what you have
> 
> [code]
...
> data_read = DATA.read
> string = "\\"
...
> __END__
> \\
> [/code]

So in this program, 'data_read' contains two backslash characters; and 
'string' contains a single backslash character.

> yeilds
> <p>\</p>
> <p>\\</p>

That looks correct to me - HTML doesn't need a backslash to be escaped. 
So now add Uv into your test to see if that is munging the backslashes.

> although I have no idea how to treat a string as a file.

A string is just a string. In ruby 1.8 it's a sequence of bytes; in ruby 
1.9 it's a sequence of characters. But that doesn't matter here; a 
backslash is a backslash, and is both a single character and a single 
byte in either ASCII or UTF-8.

However if you enter a string *literal* in a ruby program (or in IRB), 
then it is parsed with backslash escaping rules to turn it into an 
actual String object. For example:

a = "abc\ndef"
b = 'abc\ndef'

string 'a' contains 7 characters (a,b,c,newline,d,e,f), whereas string b 
contains 8 characters (a,b,c,backslash,n,d,e,f). This is because there 
are different escaping rules for double-quoted and single-quoted 
strings.

In a single-quoted string literal, \' is a single quote, and \\ is a 
backslash, and everything else is treated literally, so \n is two 
characters \ and n.

In a double-quoted string literal, \" is a double quote, \n is a 
newline, \\ is a backslash, and there's a whole load of other expansion 
including #{...} for expression interpolation and #@... for instance 
variable substitution.

> erb results are similar, which I would have though was be happening in 
> rails anyway
>>> ERB.new("\\").src
> => "_erbout = ''; _erbout.concat \"\\\\\"; _erbout"

Now you're just scaring yourself with backslash escaping :-)

Firstly, note that you passed a single backslash character to ERB. 
That's what the string literal "\\" creates.

ERB compiled it to the following Ruby code:

_erbout = ''; _erbout.concat "\\"; _erbout

which just appends a single backslash to _erbout, which is what you 
expect.

However, IRB displays the returned string from ERB.new using 
String#inspect, so it is turned into a double-quoted string. This means:
1. A " is added to the start and end of the string
2. Any " within the string is displayed as \"
3. Any \ within the string is displayed as \\

In other words, String#inspect turns a string into a Ruby string literal 
- something that you could paste directly into IRB. Try it:

>> str = "_erbout = ''; _erbout.concat \"\\\\\"; _erbout"
>> puts str

That will show you the actual contents of str, which is the Ruby code I 
pasted above.

HTH,

Brian.
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