REGEX in R: extracting words from a string

Question

i guess this is a common problem, and i found quite a lot of webpages, including some from SO, but i failed to understand how to implement it.

I am new to REGEX, and

Answer 1

For getting the first four words.

library(stringr)
str_extract(x, "^\\s*(?:\\S+\\s+){3}\\S+")

For getting the last four.

str_extract(x, "(?:\\S+\\s+){3}\\S+(?=\\s*$)")