/(?<!#|\/)\s*zone\s+"([^"]+)".+?masters\s+{[\s#\/]*([a-zA-Z0-9.-]+)[\s#\/;]*}/sg

##</code><code>##

(?<!#|\/)\s*zone
  ^  ^    ^
  |  |    |
  |  |    + - 0+ whitespaces
  |  + - Match "zone" and 0+ whitespaces that are not preceded by an "#" or "/"
  + - Negative lookbehind