There's the problem of regular expressions not dealing with matching parentheses, for one. That makes a lot of questions about code hard to express in a regexp, even something as simple as 'find foo(anything) + bar(anything)' is busted. False positives within strings are also an issue.

I did a code search thing, (which I don't claim is anything more than a hack) described over in this HN thread: http://news.ycombinator.com/item?id=4113348 but yes, the syntax wasn't quite 'a string', you'd type the code you're looking for with parts wildcarded, but parentheses in what you typed had to match.