Have you looked at perlop and perlre? Those are, probably, the definitive sources on the regex switches and options. You might also take a look at Jeffrey Friedl's book, Mastering Regular Expressions. It's a bit outdated now, since there's been a lot of new regex functionality added since the book was published; but it's still a good source.

By the way: the possible switches for the tr operator are

tr//cds
[download]