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

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