A DFA can do a non-greedy match by stopping the first time it enters an accepting state, instead of consuming input until it determines that no further matching is possible (a “jam” state). This is actually easier to implement than longest leftmost match (which flex does).
But it's also much less useful than longest leftmost match. In general, when you find yourself wishing for non-greedy matching, that's usually a sign that you're trying to make the scanner do some parsing. That's generally the wrong approach, since it lacks the power to do a decent job. Better is to either introduce a separate parser, or to split the scanner into multiple scanners using (exclusive) start conditions.
You might have a separate start state once you've seen the `BEGIN'. In that state, you might then have a regex that will match `END' (to kick you out of the state), and perhaps `(.|\n)' to get a single character within the chunk ...
This approach also has much better error-reporting properties.