Character classes represent a set of possible single-character matches. Character classes appear between square brackets [ ].
As an example, the regular expression t[aeiou]
will match "ta", "te", "ti", "to", or "tu". You can have as many character possibilities inside the square brackets as you like, but remember that any single character in the set will match. [aeiou]
looks like five characters, but it actually means "a" or "e" or "i" or "o" or "u".