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x <- "They didn't sell the company, and it went bankrupt"
gsub("(?:\\bnot|n't|\\G(?!\\A))\\s+\\K(\\w+)\\b", "not_\\1", x, perl=TRUE)
Success #stdin #stdout 0.2s 39448KB
comments (?)
stdin 
Standard input is empty
stdout 
[1] "They didn't not_sell not_the not_company, and it went bankrupt"
https://ideone.com/TWRERI
language:
R (R 3.5.2)
created:
6 years ago
visibility:
secret

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