This sub-thread effectively underscores why it's such a Good Idea to use modules. There are numerous edge cases that make this a difficult problem. Chief among them: what about empty fields? Certainly they should be possible, so you can't just grep them out. But you don't want to introduce them, either.
Then there's the issue about quoting quotes, which wasn't mentioned in this problem, but would probably come up eventually in any real-world case that gets used much. And reporting errors on malformed lines.
That said, I've got a regex that at least seems to deal with the empty fields properly:
Caution: Contents may have been coded under pressure.