R - 在字符串右侧第 n 次出现字符后提取信息

Question

我见过许多提取 w/ gsub 的迭代，但它们主要处理从左到右或出现一次后提取。我想从右到左匹配，计算 - 的四次出现次数，匹配第 3 次和第 4 次出现之间的所有内容。

例如：

string                       outcome
here-are-some-words-to-try   some
a-b-c-d-e-f-g-h-i            f

以下是我尝试使用的一些参考资料：

regex - return all before the second occurrence

Answer 1

x = c("here-are-some-words-to-try", "a-b-c-d-e-f-g-h-i")
sapply(x, function(strings){
    ind = unlist(gregexpr(pattern = "-", text = strings))
    if (length(ind) < 4){NA}
    else{substr(strings, ind[length(ind) - 3] + 1, ind[length(ind) - 2] - 1)}
})
#here-are-some-words-to-try          a-b-c-d-e-f-g-h-i 
#                    "some"                        "f"

Answer 2

你可以使用

([^-]+)(?:-[^-]+){3}$

参见a demo on regex101.com。

在 R 这可能是

library(dplyr)
library(stringr)
df <- data.frame(string = c('here-are-some-words-to-try', 'a-b-c-d-e-f-g-h-i', ' no dash in here'), stringsAsFactors = FALSE)

df <- df %>%
  mutate(outcome = str_match(string, '([^-]+)(?:-[^-]+){3}$')[,2])
df

并产生

                      string outcome
1 here-are-some-words-to-try    some
2          a-b-c-d-e-f-g-h-i       f
3            no dash in here    <NA>

Answer 3

拆分你的句子怎么样？像

string <- "here-are-some-words-to-try"

# separate all words
val <- strsplit(string, "-")[[1]]

# reverse the order
val rev(val)

# take the 4th element
val[4]

# And using a dataframe
library(tidyverse)
tibble(string = c("here-are-some-words-to-try", "a-b-c-d-e-f-g-h-i")) %>% 
mutate(outcome = map_chr(string, function(s) rev(strsplit(s, "-")[[1]])[4]))

R - 在字符串右侧第 n 次出现字符后提取信息

R - Extract info after nth occurrence of a character from the right of string

regex

string

parsing

r

gsub