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

##</code><code>##

[\s#\/]*([a-zA-Z0-9.-]+)[\s#\/;]*
   ^          ^            ^
   |          |            |
   |          |            + - Character set found immediately after IP/domain name
   |          + - Character set allowed in IP/domain name
   + - Character set found immediately before IP/domain name