Question

目标：

我想将句子与'no'一词匹配，但只有在'没有'之前没有'带'或'有'或'有'在r之前。

输入：

NSNumber

预期输出：

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

尝试：

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

问题：

负面的后视似乎被忽略，所以所有的句子都被替换了。问题是在lookbehind声明中使用交替吗？

Answer 1

你只需要一个正则表达式替换字符。想法是匹配和捕捉所有可能的＆＃34;不＆＃34;句子并匹配所有剩余的句子。然后将所有匹配的字符替换为\\1，即第一个捕获组中的字符。

gsub("(?i)(.*(with|there (?:is|are)) no\\b.*)|.*", "\\1" ,string, perl=T)

DEMO

示例：

x <- "The ground was rocky with no cracks in it\nNo diggedy, no doubt\nUnderstandably, there is no way an elephant can be green"
gsub("(?i)(.*(with|there (?:is|are)) no\\b.*\\n?)|.*\\n?", "\\1" ,x, perl=T)
# [1] "The ground was rocky with no cracks in it\nUnderstandably, there is no way an elephant can be green"

Answer 2

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 demo。简而言之：该模式找到2种替代品，两种类型的行中的一种，其中一种no整个单词，其前面没有with，there is或there are以及它们之后的空格，或包含?或:后跟0 +水平空格（\h）的行，然后是n未跟随任何其他字母的行。

请参阅R demo：

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

感谢x修饰符，您可以为正则表达式模式添加注释，并使用空格格式化它以获得更好的可读性。请注意，所有文字空格必须替换为\\h（水平空白），\\s（任何空格），\\n（LF），\\r（CR）等。让它以这种模式运作。

(?i)修饰符代表ingore.case=TRUE。

如何使用lookbehind交替

2 个答案: