I've tried a few tricky languages such as D = { w | w has an equal number of occurences of 01 and 10 as substrings} but I don't have the means to prove this one as being not regular (and I cannot confirm if it is or not). I know of the pumping lemma, which I have used on very basic languages to prove they are not regular, but I'm not even sure where to begin on this one.
