正则表达式 – 如何修剪和替换字符串

x <- "    this is a string  "
pattern <- "^ +?b|b? +$"
startstop <- gsub(" ","~",regmatches(x,gregexpr(pattern,x))[[1]])
text <- paste(regmatches(x,x),invert=TRUE)[[1]],collapse="")
paste0(startstop[1],text,startstop[2])
# [1] "~~~~this is a string~~"

而且,为了好玩,作为一个功能,以及一个“矢量化”功能：

## The function
replaceEnds <- function(string) {
  pattern <- "^ +?b|b? +$"
  startstop <- gsub(" ",regmatches(string,string))[[1]])
  text <- paste(regmatches(string,string),invert = TRUE)[[1]],collapse = "")
  paste0(startstop[1],startstop[2])
}

## use Vectorize here if you want to apply over a vector
vReplaceEnds <- Vectorize(replaceEnds)

一些样本数据：

myStrings <- c("    Four at the start,2 at the end  ","   three at the start,one at the end ")

vReplaceEnds(myStrings)
#        Four at the start,2 at the end        three at the start,one at the end  
#  "~~~~Four at the start,2 at the end~~" "~~~three at the start,one at the end~"