如何使用 stringr 的 replace_all() 函数替换字符串中的特定匹配项 [英] How to use stringr's replace_all() function to replace specific matches in a string

问题描述

stringr 包有帮助 str_replace() 和 str_replace_all() 函数.例如

The stringr package has helpful str_replace() and str_replace_all() functions. For example

mystring <- "one fish two fish red fish blue fish"

str_replace(mystring, "fish", "dog") # replaces the first occurrence
str_replace_all(mystring, "fish", "dog") # replaces all occurrences

棒极了.但是你如何

1. 替换第二次出现的fish"?
2. 替换最后一次出现的fish"?
3. 替换倒数第二次出现的鱼"?

推荐答案

对于第一个和最后一个，我们可以使用 stringi 中的 stri_replace 因为它有选项

For the first and last, we can use stri_replace from stringi as it has the option

 library(stringi)
 stri_replace(mystring, fixed="fish", "dog", mode="first")
 #[1] "one dog two fish red fish blue fish"

 stri_replace(mystring, fixed="fish", "dog", mode="last")
 #[1] "one fish two fish red fish blue dog"

mode 只能有值 'first'、'last' 和 'all'.因此，其他选项不在默认功能中.我们可能必须使用 regex 选项来更改它.

The mode can only have values 'first', 'last' and 'all'. So, other options are not in the default function. We may have to use regex option to change it.

使用sub，我们可以对单词进行第n次替换

Using sub, we can do the nth replacement of word

sub("^((?:(?!fish).)*fish(?:(?!fish).)*)fish", 
           "\\1dog", mystring, perl=TRUE)
#[1] "one fish two dog red fish blue fish"

或者我们可以使用

 sub('^((.*?fish.*?){2})fish', "\\1\\dog", mystring, perl=TRUE)
 #[1] "one fish two fish red dog blue fish"

为了简单起见，我们可以创建一个函数来执行此操作

Just for easiness, we can create a function to do this

patfn <- function(n){
 stopifnot(n>1)
 sprintf("^((.*?\\bfish\\b.*?){%d})\\bfish\\b", n-1)
} 

并替换第n次出现的'fish'，除了第一个可以使用sub或str_replace

and replace the nth occurrence of 'fish' except the first one which can be easily done using sub or the default option in str_replace

sub(patfn(2), "\\1dog", mystring, perl=TRUE)
#[1] "one fish two dog red fish blue fish"
sub(patfn(3), "\\1dog", mystring, perl=TRUE)
#[1] "one fish two fish red dog blue fish"
sub(patfn(4), "\\1dog", mystring, perl=TRUE)
#[1] "one fish two fish red fish blue dog"

这也适用于 str_replace

 str_replace(mystring, patfn(2), "\\1dog")
 #[1] "one fish two dog red fish blue fish"
 str_replace(mystring, patfn(3), "\\1dog")
 #[1] "one fish two fish red dog blue fish"

<小时>

基于上面提到的模式/替换，我们可以创建一个新函数来完成大部分选项

---

Based on the pattern/replacement mentioned above, we can create a new function to do most of the options

replacerFn <- function(String, word, rword, n){
 stopifnot(n >0)
  pat <- sprintf(paste0("^((.*?\\b", word, "\\b.*?){%d})\\b",
           word,"\\b"), n-1)
  rpat <- paste0("\\1", rword)
  if(n >1) { 
    stringr::str_replace(String, pat, rpat)
   } else {
    stringr::str_replace(String, word, rword)
    }
 }


 replacerFn(mystring, "fish", "dog", 1)
 #[1] "one dog two fish red fish blue fish"
 replacerFn(mystring, "fish", "dog", 2)
 #[1] "one fish two dog red fish blue fish"
 replacerFn(mystring, "fish", "dog", 3)
 #[1] "one fish two fish red dog blue fish"
 replacerFn(mystring, "fish", "dog", 4)
 #[1] "one fish two fish red fish blue dog"

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