day 07: Use positive lookahead on 0-length matches
day 07: Use positive lookahead on 0-length matches

This allows scan to find overlapping matches, interstingly.

From reddit:
https://www.reddit.com/r/adventofcode/comments/5gy1f2/_/daw0e69/

petertseng/adventofcode-rb-2016,petertseng/adventofcode-rb-2016