Learning Rails

Character Classes

Example C-1 tested against letters and numbers, but there are many ways to do that. [a-z] is a good way to test for lowercase letters in English, but many languages use characters outside of that range. Regular expression character classes let you create sets of characters as well as use predefined groups of characters to identify what you want to target.

To create your own character class, use the square braces: [ and ]. Within the square braces, you can either list the characters you want, or create a set of characters with the hyphen. To match all the (guaranteed) English vowels in lowercase, you would write:

/[aeiou]/

If you wanted to match both upper- and lowercase vowels, you could write:

/[aeiouAEIOU]/

(If you wanted to ignore case entirely in your search, you could also use the i modifier described earlier: /[aeiou]/i.)

You can also mix character classes in with other parts of a search:

/[Rr][aeiou]by/

That would match Ruby, ruby, raby, roby, and a lot of other variations with upper- or lowercase R, followed by a lowercase vowel, followed by by.

Sometimes listing all the characters in a class is a hassle. Regular expressions are difficult enough to read without huge chunks of characters in classes. So instead of:

/[abcdefghijklmnopqrstuvwxyz]/

you can just write:

/[a-z]/

As long as the characters you want to match form a single range, that’s simple—the hyphen just means “everything in between.”

There’s also a “not” option available, in the ^ character. You can reverse ...