Jim Hoskins

I wanted to post a quick guide to the special ruby syntax for literals that utilize the % (percent) symbol. Most beginners guides to ruby leave out an explanation of these forms of literals, but many ruby coders use them. When someone encounters them for the first time it is almost impossible to figure out what they mean. (Try searching Google for “%w”)

Ruby has special syntax for making strings, arrays and system commands easier to write. They allow you to use different characters as delimiters so you can minimize escaping in your literals.

The syntax

The syntax for the % literals is a percent symbol (%) a letter which defines what kind of literal it is (Q, q, w, x, r) a delimiter,  the content, and the closing delimiter.

The delimiter can be any character, and is defined as whatever is immediately after the letter in the syntax. For example %Q!content! , the delimiter is the ! and it surrounds the content. There are special cases when the delimiter is { or (, the closing delimiter will be } or ) respectively.

%Q and %q (Percent Q): Strings

%Q!Some String of “Characters”! <==> ” Some String of \”Characters\” “

%Q is the equivalent to a double-quoted ruby string. #{expression} evaluation works just like in double-quoted strings, even if you use %Q{} as your delimiter!

You can also leave off the Q and it will have the same functionality. I recommend leaving the Q in to be more clear.

%q!Some String of “Characters”! <==> ‘Some String of Characters’

The %q is just like %Q, but acts the same as a single-quoted string. Whatever is inside the delimiters is returned as a string.

You can remember %Q is for strings because it acts like Quotes.

More info here: http://docs.huihoo.com/ruby/ruby-man-1.4/syntax.html#string

%W (Percent W): Arrays

%W(North South East West) <==> ["North", "South", "East", "West"]

%W (and %w) allow you to create an Array of strings without using quotes and commas.

The delimiter rules are the same as strings, but typically parentheses are used. The content inside the delimiters are split by white-space, and put into an array. This is great if you have a hard coded list of single word strings.

When using %W (capital W), it is evaluated as a double-quoted string. This allows you to use #{} to interpolate values. %w (lower-case w) will evaluate as a single quoted string.

You can remember %W is by thinking of it as a White-space divided Array.

More info here : http://docs.huihoo.com/ruby/ruby-man-1.4/syntax.html#array

%x (Percent x): System Execution

%x{ ls /usr/local } <==> `ls /usr/local`

%x allows you to call system commands, equivilent to wrapping the command in `s (grave accents). The benefit of the $x{} syntax is you don’t have to escape your accents in commands that use them.

You can remember to use X because it eXecutes a command.

More info here: http://docs.huihoo.com/ruby/ruby-man-1.4/syntax.html#command

%r (Percent r): Regular Expressions

%r{/usr/bin/} <==> /\/usr\/bin\//

%r is really handy for regular expressions that contain /s (forward slashes) which are the default delimiter for regular expressions and have to be escaped.

Remember to use %r with regular expressions.

More info here: http://docs.huihoo.com/ruby/ruby-man-1.4/syntax.html#regexp

I hope this information is helpful. Please leave a comment if this helped or if I left something out.

Tags: Percent Functions, Percent Literals, Ruby, Syntax

This entry was posted on Tuesday, October 7th, 2008 at 8:47 pm and is filed under Ruby, Tutorial. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.