What is the difference between %w and %W array literals in Ruby

This minipost will explain in detail the difference between the lower case %w and the uppercase %W percent string literals in Ruby on string interpolation cases and escape sequences.

By definition the lower case %w string literal creates an Array of String-s if you supply words separated by whitespaces inside the brackets.

The following declaration:

%w[a b c d]

will output:

["a", "b", "c", "d']

However, the %w literal treat the supplied strings as single-quoted and as a result, it constructs an Array of strings that have not been escaped or interpolated.

To demonstrate most of the cases and the main difference between %w and %W Ruby Array percent literals we will assume the following for the irb session:

a = 'a'

We will execute a %w construction that contains interpolation, escape sequences and the special case of adding whitespace between two strings.

The following declaration:

%w[a#{a} b#{'b'} c\ d \s \']

will output:

=> ["a\#{a}", "b\#{'b'}", "c d", "\\s", "\\'"]

From the above we can notice the following:

  • that we tried to interpolate a String with a variable and a String with another String, however, all the interpolation declaration was not escaped and it evaluated in a string with value "a\#{a}" and in a string with "b\#{'b'}" as the first two items in the Array.
  • the lower case %w literal only adds whitespaces between the supplied strings if and only if you have specified \ (backward slash & whitespace) and not the space literal \s


On the other hand, the upper-case %W literal treat the supplied strings in the brackets as double-quoted String-s and thus it will construct an Array from interpolated string expressions and it will escape sequences prior to the Array creation.

To inspect the behavior of the the %W literal we will use the previous example:

%W[a#{a} b#{'b'} c\ d \s \']

and inspect the output:

=> ["aa", "bb", "c d", " ", "'"]

From the above snippet, we can see that %W percent literal allows string interpolation between a String and a dynamic variable, it escapes sequences and the \ (backward slash & space) sequence that the lower case uses for spaces between Strings.


The lower-case %w percent literal:

  • treats all supplied words in the brackets as single-quoted Strings
  • does not interpolate Strings
  • does not escape sequences
  • escapes only whitespaces by \


The upper-case %W percent literal:

  • treats all supplied words in the brackets as double-quoted Strings
  • allows String interpolation
  • escapes sequences
  • escapes also whitespaces by \
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