But if you really do want to use a Regex the following will work and does so for the special cases of one or two letter words.

s/\b\w|\w\b/\U$&\E/g
[download]

or if you don't want "toasted weasels" to go to "ToasteD WeaselS" but "Toasted weaselS" then:

s/\A\w|\w\z/\U$&\E/g
[download]

Dedalus.