Before, you could describe the set of all digits using [0-9]. Because some sets are used regularly, there are short notations for them.

• \d is the set of add digits.
• \s is the set of all spaces, e.g. or the line
  break.
• . matches all characters except the line break. .* can be used if you do not care about some content.
• \w is for letters in words. \w is the same as [0-9a-zA-Z]_.
• \\ matches the backslash \.

The following text is from "The Twelve Days of Christmas" by Harry Belafonte [source]. Here, animals are counted. Find all lines in which a number is followed by space followed by a word.