你可以使用lookahead来Assert一个答案后面没有一个具有相同键的Key:行：

^                           # Match an answer that starts at the start of line,
(?<key>[ABCD])              # then a key, which we capture,
\.\s+.+                     # a dot, some spaces and everything else to the end of line,
(?=                         # followed by
  (?:\n[ABCD]\.\s+.+){0,3}  # 0 to 3 more answers, then
  \n                        # a blank line,
  \nKey:\s+                 # then 'Key:' succeeded by some spaces and
  (?!\k<key>)               # something that is not the same as the key we captured.
)

同样的正则表达式也可以在Python中使用，只是在语法上有一些细微的差异：

^(?P<key>[ABCD])\.\s+.+
(?=
  (?:\n[ABCD]\.\s+.+){0,3}
  \n
  \nKey:\s+(?!(?P=key))
)

在www.example.com上试试regex101.com：PCRE/PCRE2/Java 8/.NET，ECMAScript，Python。