On of my pet projects is a word segmenter, meaning a programm which gets sentences and spits out words. Easy to get good results for the English language, but pretty hard for languages like Japanese were you won't find any spaces between words. A pretty though task were academic and commercial research aren't that advanced.

I have to say for this task Perl is the perfect language. I don't care on speed but only on results. If I get satisfied by my project I may implement it in C, or may not. Call it prototyping, call it research, call it whatever, I won't try it with Prolog or Lisp.

Hanamaki