A character set is a group of characters from which only one is desired
[0123456789] – matches any single number
Sets can use ranges of characters (think ascii table)
[0-9] – matches any single character
A dash can be represented in a set by placing it first (I.e. not in a range)
[-aeiou] – matches a dash or a vowel
A Carat (^) at the beginning of a set negates if (I.e. anything BUT characters in the set