Question

I’ve been working my way through the Ruby Koans and am confused by the “escape clauses and single quoted strings” examples.

One example shows that I can’t really use escape characters in this way, but immediately after, the following example is given:

def test_single_quotes_sometimes_interpret_escape_characters
  string = '\\\''
  assert_equal 2, string.size # <-- my answer is correct according to the program
  assert_equal "\\'", string  # <-- my answer is correct according to the program
end

This has confused me on two fronts:

1. Single quotes can sometimes be used with escape characters.
2. Why is the string size 2, when assert_equal is "\\\'"? (I personally thought the answer was "\'", which would make more sense with regards to size).

Answer 1

You can break your string into two pieces to clarify things:

string = '\\' + '\''

Each part is a string of length one; '\\' is the single character \ and '\'' is the single character '. When you put them together you get the two character string \'.

There are two characters that are special within a single quoted string literal: the backslash and the single quote itself. The single quote character is, of course, used to delimit the string so you need something special to get a single quote into a single quoted string, the something special is the backslash so '\'' is a single quoted string literal that represents a string containing one single quote character. Similarly, if you need to get a backslash into a single quoted string literal you escape it with another backslash so '\\' has length one and contains one backslash.

The single quote character has no special meaning within a double quoted string literal so you can say "'" without any difficulty. The backslash, however, does have a special meaning in double quoted strings so you have to say "\\" to get a single backslash into your double quoted string.

Consider your guess off "\'". The single quote has no special meaning within a double quoted string and escaping something that doesn’t need escaping just gives you your something back; so, if c is a character that doesn’t need to be escaped within a double quoted string, then \c will be just c. In particular, "\'" evaluates to "'" (i.e. one single quote within a double quoted string).

The result is that:

• '\\\'' == "\\'"
• "\\\"" == '\\"'
• "\'" == '\''
• "\'" == "'"
• '\\\''.length == 2
• "\\\"".length == 2
• "\'".length == 1
• "'".length == 1

The Wikibooks reference that Kassym gave covers these things.

I usually switch to %q{} (similar to single quoting) or %Q{} (similar to double quoting) when I need to get quotes into strings, all the backslashes make my eyes bleed.