Backslashes in single quoted strings vs. double quoted strings

Question

If I add a backslash+space to the start of double and single quoted strings, I get different results:

"\ text"
'\ text' 

In the output for the double quoted string I see only a space.
In the output for the single quoted string I see backslash+space.

What's happening there? Is this because '\ ' is interpreted as a special character in the double quote string but in the single quoted string the characters are preserved as is?

If I change the strings to this, I see the same output, namely a single slash followed by a space and then the text:

"\\ text"
'\\ text' 

In both cases the backslash is escaped. I'm confused why they work the same way in this situation.

Is there some rule that would help to explain the fundamental difference between how single quoted strings and double quoted strings handle backslashes in Ruby?

Answer 1

I'd refer you to "Ruby Programming/Strings" for a very concise yet comprehensive overview of the differences.

From the reference:

puts "Betty's pie shop"

puts 'Betty\'s pie shop'

Because "Betty's" contains an apostrophe, which is the same character as the single quote, in the second line we need to use a backslash to escape the apostrophe so that Ruby understands that the apostrophe is in the string literal instead of marking the end of the string literal. The backslash followed by the single quote is called an escape sequence.

Answer 2

Double-quoted strings support the full range of escape sequences, as shown below:

• \a Bell/alert (0x07)
• \b Backspace (0x08)
• \e Escape (0x1b)
• \f Formford (0x0c)
• \n Newline (0x0a)
• \r Return (0x0d)
• \s Space (0x20)
• \t Tab (0x09)
• \v Vertical tab (0x0b)

For single-quoted strings, two consecutive backslashes are replaced by a single backslash, and a backslash followed by a single quote becomes a single quote:

'escape using "\\"' -> escape using "\"
'That\'s right'     -> That's right

Answer 3

Ruby only interprets escape sequences in double quoted strings. In a single quoted string, only \\ (backslash backslash) and \' (backslash quote) are taken as special characters. You should use double quoted strings only when you need more interpretation. Otherwise, single quotes provide a performance boost.

When you mentioned including the name of a variable, Ruby never does that. Just the variable name is treated as more of the string literal. To include the value of a variable (or any expression) put the expression in like this:

"#{variable}"

Note that this only works in double quoted strings. To add a variable to a single quoted one, you need to do this:

'The value of X is: '+X

If you need serious formatting, look into Ruby's version of sprintf and printf. They are pretty much wrappers around the C functions, and are quite powerful, but a bit cumbersome to use.

Answer 4

This is not a full answer (since the simple question has been answered already), but rather it is supplementary information.

Which style of Ruby string quoting do you favour?

Don't use double quotes if you have to escape them. And don't fall in "single vs double quotes" trap. Ruby has excellent support for arbitrary delimiters for string literals:

http://rors.org/2008/10/26/dont-escape-in-strings

I took that advice and have never looked back!

Answer 5

Is this because the '\ ' is interpreted as a special character in the double quote string but in the single quoted string the characters are preserved as is?

Yes. Single-quoted strings are treated as literals; double-quoted strings are interpolated. This is the same in other Ruby-like languages, and hasn't changed in 1.9.

Answer 6

you can make <%= f.label :nom_entreprise, "Nom de l'entreprise" %>