library(stringr)
test <- "asdf asiodjfojewl kjwnkjwnefkjnkf [asdf] fasdfads fewrw [keyword<1] keyword [keyword>1]"
## If the word is right after "[":
str_extract_all(test, "(?<=\\[)keyword[^\\]\\[]*(?=])")
## If the word is anywhere betwee "[" and "]":
str_extract_all(test, "(?<=\\[)[^\\]\\[]*?keyword[^\\]\\[]*(?=])")