Also cannot solve the "dangling else" problem. That is, if you're checking a piece of code with a bunch of "if" statements which may or may not be followed by "else" statements, it is impossible to write a regexp that will find all occurances of "elses" without matching "ifs" (e.g. the point at which the number of elses exceeds the number of ifs). Note that this is trivially easy to do with a loop.

There are a bunch more things also; have to dig up my compiler books to list them, though :)