如何使用后向交替

您可以使用

(?mxi)^       # Start of a line (and free-spacing/case insensitive modes are on)
(?:           # Outer container group start
  (?!.*\b(?:with|there\h(?:is|are))\h+no\b) # no 'with/there is/are no' before 'no'
  .*\bno\b  # 'no' whole word after 0+ chars
  (?![?:])    # cannot be followed with ? or :
|             # or
  .*          # any 0+ chars
  [?:]\h*n(?![a-z]) # ? or : followed with 0+ spaces, 'n' not followed with any letter
)             # container group end
.*            # the rest of the line and 
\R*           # 0+ line breaks

请参阅regex演示。简而言之：将图案发现2个选择，不是的2种类型的线中，一种具有no在它整个字未前面有with，there is或there are和它们之后的空间中，或含有线?或:随后与0+水平空间（\h）然后再加上一个n不带其他字母的字母。

参见R演示：

sentences <- "The ground was rocky with no cracks in it\r\nNo diggedy, no doubt\r\nUnderstandably, there is no way an elephant can be green"
rx <- "(?mxi)^ # Start of a line
(?:            # Outer container group start
  (?!.*\\b(?:with|there\\h(?:is|are))\\h+no\\b) # no 'with/there is/are no' before 'no'
  .*\\bno\\b   # 'no' whole word after 0+ chars
  (?![?:])     # cannot be followed with ? or :
|              # or
  .*           # any 0+ chars
  [?:]\\h*n(?![a-z]) # ? or : followed with 0+ spaces, 'n' not followed with any letter
)              # container group end
.*             # the rest of the line and 0+ line breaks
\\R*"
res <- gsub(rx, "", sentences, perl=TRUE)
cat(res, sep="\n")

输出：

The ground was rocky with no cracks in it
Understandably, there is no way an elephant can be green

多亏了x修饰符，您可以在正则表达式模式中添加注释，并使用空格对其进行格式化以提高可读性。请注意，所有文字空白都必须替换为\\h（水平空白），\\s（任何空白），\\n（LF），\\r（CR）等，以使其以这种模式工作。

该(?i)修改代表的ingore.case=TRUE。