combine any word which comes after specific word - r

Question

I want to combine word which comes after a specific word ,I have try bigram approach which is too slow and also tried with gregexpr but didnt get any good solution. for ex
text="This approach isnt good enough."
BigramTokenizer <- function(x) NGramTokenizer(x, Weka_control(min = 2, max = 2))
BigramTokenizer(text)
[1] "This approach" "approach isnt" "isnt good" "good enough"
what i really want is isnt_good as single word in a text ,combine next word which comes after isnt.
text
"This approach isnt_good enough."
Any efficient approach to convert into unigram.Thanks.

Answer 1

To extract all occurrences of the word "isn't" and the following word you can do this:
library(stringr)
pattern <- "isnt \\w+"
str_extract_all(text, pattern)
[[1]]
[1] "isnt good"
It essentially does the same thing as the example below (from the base package) but I find the stringr solution more elegant and readable.
> regmatches(text, regexpr(pattern, text))
[1] "isnt good"
Update
To replace the occurrences of isnt x with isnt_x you just need gsub of the base package.
gsub("isnt (\\w+)", "isnt_\\1", text)
[1] "This approach isnt_good enough."
What you do is to use a capturing group that copies whatever is found inside the parentheses to the \\1. See this page for a good introduction: http://www.regular-expressions.info/brackets.html

Answer 2

How about this function?
joinWords <- function(string, word){
y <- paste0(word, " ")
x <- unlist(strsplit(string, y))
paste0(x[1], word, "_", x[2])
}
> text <- "This approach isnt good enough."
> joinWords(text, "isnt")
# [1] "This approach isnt_good enough."
> joinWords("This approach might work for you", "might")
# [1] "This approach might_work for you"