Extracts a certain number of words from a text string.

wordExtract.R

wordExtract <- function(instring, number, start = TRUE, after = NULL) {
  len <- length(gregexpr("\\S+", instring)[[1]])
  mlen <- if (is.null(after)) number else number + after
  if (len <= mlen) stop("can't do what you've asked for....")

  if (!is.null(after) & !isTRUE(start)) {
    start <- TRUE
    message("start specified as FALSE but ignored")
  } 
  
  if (!is.null(after)) {
    pat <- "^(?:\\S+\\s+){%d}((?:\\S+\\s+){%d}\\S+).*"
    pat <- sprintf(pat, after, number-1)
  } else {
    pat <- if (isTRUE(start)) "^((?:\\S+\\s+){%d}\\S+).*"
    else "^.*\\s+((?:\\S+\\s+){%d}\\S+)$"
    
    pat <- sprintf(pat, number-1)
  }
  sub(pat, "\\1", instring, perl = TRUE)
}

Example:

x <- "this is a string that I want to extract words from"
wordExtract(x, 3)
# [1] "this is a"
wordExtract(x, 3, FALSE)
# [1] "extract words from"
wordExtract(x, 3, after = 5)
# [1] "I want to"