R中的负面后视与多词分离 [英] Negative lookbehind in R with multi-word separation

问题描述

我使用R来进行一些字符串处理，并且想要识别具有某个词根的字符串，而这些字符串不是由某个词根的另一个词语开头的。

这是一个简单的玩具示例。假设我想识别字符串中任何字符串中没有dog / s的单词cat / s。

 测试= c（
狗猫，
狗和猫，
狗和猫，
狗和蓬松的猫，
猫和狗，
猫和狗，
蓬松的猫和蓬松的狗） 
  

使用这个模式，我可以把 cat：

  pattern =（dog（s |）。*）（cat（s |））
 grep（pattern，tests，perl = TRUE，value = TRUE）
 
 [1]dog catdog and catsdog and catdog and fluffy cats
  

我的负面lookbehind存在问题：

  neg_pattern =（？<！dog（s |）。*）（cat（s |））
 grep（neg_pattern，tests，perl = TRUE，value = TRUE）
  

grep中的错误（neg_pattern，tests，perl = TRUE，value = TRUE）：
无效正则表达式

另外：警告消息：
在grep（neg_pattern，tests，perl = TRUE ，value = TRUE）：
PCRE模式编译错误
'lookbehind断言不是固定长度'
at'）（cat（s |））'

据我所知，*不是固定长度，所以我怎样才能拒绝在cat之前有任何其他单词分隔的dog的字符串？

解决方案

我希望这可以帮助您：

 <$ c 
狗和猫，
狗和猫，
狗和蓬松的猫，
狗猫 b猫和狗，
猫和狗，
蓬松的猫和蓬松的狗
）
 
＃删除有狗后有猫的琴弦
 tests = tests [-grep（pattern =dog（？：s |）。* cat（？：s |），x = tests）] 
 
＃只选择包含cats 
 tests = tests [grep（pattern =cat（？：s |），x = tests）] 
 
 tests 
 $ b [1]cats和狗猫和狗
 [3]蓬松的猫和蓬松的狗
  

我不确定您是否想用一个表达式来完成此操作，但是当迭代应用时，
Regex仍然非常有用。 / p>

I'm using R to do some string processing, and would like to identify the strings that have a certain word root that are not preceded by another word of a certain word root.

Here is a simple toy example. Say I would like to identify the strings that have the word "cat/s" not preceded by "dog/s" anywhere in the string.

 tests = c(
   "dog cat",
   "dogs and cats",
   "dog and cat", 
   "dog and fluffy cats",
   "cats and dogs", 
   "cat and dog",  
   "fluffy cats and fluffy dogs")  

Using this pattern, I can pull the strings that do have dog before cat:

 pattern = "(dog(s|).*)(cat(s|))"
 grep(pattern, tests, perl = TRUE, value = TRUE)

[1] "dog cat"  "dogs and cats"   "dog and cat"   "dog and fluffy cats"

My negative lookbehind is having problems:

 neg_pattern = "(?<!dog(s|).*)(cat(s|))"
 grep(neg_pattern, tests, perl = TRUE, value = TRUE)

Error in grep(neg_pattern, tests, perl = TRUE, value = TRUE) : invalid regular expression

In addition: Warning message: In grep(neg_pattern, tests, perl = TRUE, value = TRUE) : PCRE pattern compilation error 'lookbehind assertion is not fixed length' at ')(cat(s|))'

I understand that .* is not fixed length, so how can I reject strings that have "dog" before "cat" separated by any number of other words?

解决方案

I hope that this can help:

tests = c(
  "dog cat",
  "dogs and cats",
  "dog and cat", 
  "dog and fluffy cats",
  "cats and dogs", 
  "cat and dog",  
  "fluffy cats and fluffy dogs"
)

# remove strings that have cats after dogs
tests = tests[-grep(pattern = "dog(?:s|).*cat(?:s|)", x = tests)]

# select only strings that contain cats
tests = tests[grep(pattern = "cat(?:s|)", x = tests)]

tests

[1] "cats and dogs"               "cat and dog"                
[3] "fluffy cats and fluffy dogs"

I'm not sure if you wanted to do this with one expression, but Regex can still be very useful when applied iteratively.

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