The Ruby Way: Solutions and Techniques in Ruby Programming, Second Edition

3.8. Using Character Classes

Character classes are simply a form of alternation (specification of alternative possibilities) where each submatch is a single character. In the simplest case, we list a set of characters inside square brackets:

/[aeiou]/    # Match any single letter a, e, i, o, u; equivalent
             # to /(a|e|i|o|u)/ except for group-capture

Inside a character class, escape sequences such as \n are still meaningful, but metacharacters such as . and ? do not have any special meanings:

/[.\n?]/     # Match any of: period, newline, question mark

The caret (^) has special meaning inside a character class if used at the beginning; it negates the list of characters (or refers to their complement):

[^aeiou]     # Any character EXCEPT a, e, i, o, u

The hyphen, ...