Try the following regex:

/zone\s+"([^"]+)".+masters\s+{[\s#\/]*([a-zA-Z0-9.-]+)[\s#\/;]*}/s
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In addition to accommodating for the # and / characters in the zone stanzas, it handles the ; at the end of the masters' IP/domain name.

The last character sets:

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