helipacter helipacter - 1 year ago 76
Ruby Question

How to use escape characters in strings

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

This has confused me on two fronts:

  1. Single quotes can sometimes be used with escape characters.

  2. Why is the string size
    , when
    ? (I personally thought the answer was
    , which would make more sense with regards to size).

Answer Source

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.

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