[andrewem]

Doug McIlroy's solution is genius, and I hesitate to try to improve it, but it seems to me that if you reverse the order of the 'tr' commands, then you can slightly simplify the one that turns non-word characters into newlines, like so:

tr A-Z a-z | tr -cs a-z '\n' | ...

Since you first turn upper-case characters into lower-case ones, you then get to replace only things that aren't in a-z with newlines, rather than things that aren't in A-Za-z. It saves a whopping three characters!