您可以将((?:\d+, ){3}\d),替换为\1\n
您基本上捕获了group1中的所有内容，直到第四个逗号，然后将其替换为\1\n，\1\n将匹配的文本替换为group1文本和换行符，从而得到预期的结果。

• • 一个
• • 第一个e第一个f第一个x

gsub("((?:\\d+, ){3}\\d),", "\\1\n", "1, 2, 3, 4, 5, 6, 7, 8, 9, 10")

指纹，

[1] "1, 2, 3, 4\n 5, 6, 7, 8\n 9, 10"

• • 编辑：**

为了将上述解决方案推广到任何文本，我们可以将\d更改为[^,]

• • 一个月一次**

gsub("((?:[^,]+, ){3}[^,]+),", "\\1\n", "1, 2, 3, 4, 5, 6, 7, 8, 9, 10")
gsub("((?:[^,]+, ){3}[^,]+),", "\\1\n", "a, bb, ccc, dddd, 500, 600, 700, 800, 900, 1000")

输出，

[1] "1, 2, 3, 4\n 5, 6, 7, 8\n 9, 10"
[1] "a, bb, ccc, dddd\n 500, 600, 700, 800\n 900, 1000"

regmatches作为另一个替换：

a <- "1, 2, 3, 4, 5, 6, 7, 8, 9, 10"

fn <- ","
rp <- "\n"
n <- 4

regmatches(a, gregexpr(fn, a)) <- list(c(rep(fn,n-1),rp))
a
#[1] "1, 2, 3, 4\n 5, 6, 7, 8\n 9, 10"

作为函数：

a <- "1, 2, 3, 4, 5, 6, 7, 8, 9, 10"

replN <- function(x, fn, rp, n) {
    regmatches(x, gregexpr(fn, x)) <- list(c(rep(fn,n-1),rp))
    x
}
replN(a, ",", "\n", 4)
#[1] "1, 2, 3, 4\n 5, 6, 7, 8\n 9, 10

你甚至可以扩展它，通过replacement参数进行向量化：

a = "1, 2, 3, 4, 5, 6, 7, 8, 9, 10"

replN <- function(x,fn,rp,n) {
    sel <- rep(fn, n*length(rp))
    sel[seq_along(rp)*n] <- rp
    regmatches(x, gregexpr(fn, x)) <- list(sel)
    x
}
replN(a, fn=",", rp=c("1st","2nd"), n=4)
#[1] "1, 2, 3, 41st 5, 6, 7, 82nd 9, 10"

同时使用regex和gsub。

a = paste(1:10,collapse=", ")
x <- gsub("([^,]*,[^,]*,[^,]*,[^,]*),", '\\1\n', a)
x
#> [1] "1, 2, 3, 4\n 5, 6, 7, 8\n 9, 10"

regex是最好的选择，尽管如此，这里还有另一种不使用regex的方法

> str_vec <- strsplit(a, " ")[[1]] 
> where <- seq_along(str_vec) %% 4 == 0
> str_vec[where] <- sub(",", "\n", str_vec[where])
> paste(str_vec, collapse=" ")
[1] "1, 2, 3, 4\n 5, 6, 7, 8\n 9, 10"

这个可以用字符串代替字符。我做了一个函数，你可以很容易地使用：）
A demo here to understand the regex

> a = paste(1:10,collapse=", ")
> a
[1] "1, 2, 3, 4, 5, 6, 7, 8, 9, 10"
> # if you want the 2nd occurence
> gsub("(.*?,.*?),(.*)", "\\1\n\\2", a)
[1] "1, 2\n 3, 4, 5, 6, 7, 8, 9, 10"
> # if you want the 3rd occurence
> gsub("(.*?,.*?,.*?),(.*)", "\\1\n\\2", a)
[1] "1, 2, 3\n 4, 5, 6, 7, 8, 9, 10"
> # if you want the 4rd occurence
> gsub("(.*?,.*?,.*?,.*?),(.*)", "\\1\n\\2", a)
[1] "1, 2, 3, 4\n 5, 6, 7, 8, 9, 10"
> # if you want the last occurence
> gsub("(.*,.*),(.*)", "\\1\n\\2", a)
[1] "1, 2, 3, 4, 5, 6, 7, 8, 9\n 10"
> 
> 
> replace.occurence <- function(x, pattern, replacement, which.occu) {
+   if( which.occu == "last" ) {
+     gsub(paste0("(.*", pattern, ".*)", pattern, "(.*)"), paste0("\\1", replacement, "\\2"), x)
+   } else {
+     gsub(paste0("(.*?", paste0(rep(paste0(pattern, ".*?"), which.occu - 1), collapse = ""), ")", pattern, "(.*)"), paste0("\\1", replacement, "\\2"), x)
+   }
+ }
> 
> replace.occurence(a, pattern = ",", replacement = "\n", which.occu = 2)
[1] "1, 2\n 3, 4, 5, 6, 7, 8, 9, 10"
> replace.occurence(a, pattern = ",", replacement = "\n", which.occu = 3)
[1] "1, 2, 3\n 4, 5, 6, 7, 8, 9, 10"
> replace.occurence(a, pattern = ",", replacement = "\n", which.occu = 4)
[1] "1, 2, 3, 4\n 5, 6, 7, 8, 9, 10"
> replace.occurence(a, pattern = ",", replacement = "\n", which.occu = "last")
[1] "1, 2, 3, 4, 5, 6, 7, 8, 9\n 10"
> 
> replace.occurence(a, pattern = ", 3, 4,", replacement = ", 4, 3,", which.occu = 1)
[1] "1, 2, 4, 3, 5, 6, 7, 8, 9, 10"