I'd love to see a discussion of section 5.4.3.4 of Programming Perl (3rd Edition) The Camel Book. The section is called, "Defining your own character properties", and the text makes the following assertion:

Perl itself uses exactly the same tricks to define the meanings of its "classic" character classes (like \w) when you include them in your own custom character classes (like [-.\w\s]).

I'd love to learn a new trick, and can't really make heads or tails of what that section is discussing. ;)