How to trim and replace a string

Question

string<-c("       this is a string  ")

Is it possible to trim-off the white spaces on both the sides of the string (or just one side as required) and replace it with a desired character, such as this, in R? The number of white spaces differ on each side of the string and have to be retained on replacement.

"~~~~~~~this is a string~~"

Answer 1

This seems like an inefficient way of doing it, but maybe you should be looking in the direction of gregexpr and regmatches instead of gsub:

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

And, for fun, as a function, and a "vectorized" function:

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

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

Some sample data:

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~"

Answer 2

Use gsub:

gsub(" ", "~", "    this is a string  ")
[1] "~~~~this~is~a~string~~"

This function uses regular expressions to replace (i.e. sub), all occurrences of a pattern inside a string.

In your case, you have to express the pattern in a special way:

gsub("(^ *)|( *$)", "~~~", "    this is a string  ")
[1] "~~~this is a string~~~"

The pattern means:

• (^ *): Find one or more spaces at the start of the string
• ( *$): Find one or more spaces at the end of the string
• `|: The OR operator

---

Now you can use this approach to tackle your problem of replacing each space with a new character:

txt <- "    this is a string  "
foo <- function(x, new="~"){
  lead <- gsub("(^ *).*", "\\1", x)
  last <- gsub(".*?( *$)", "\\1", x)
  mid  <- gsub("(^ *)|( *$)", "", x)
  paste0(
    gsub(" ", new, lead),
    mid,
    gsub(" ", new, last)
  )
}

> foo("    this is a string  ")
[1] "~~~~this is a string~~"

> foo(" And another one        ")
[1] "~And another one~~~~~~~~"

For more, see ?gsub or ?regexp.