Change text to lowercase in R keeping acronyms in uppercase in text mining

Question

How can I change a full text to lowercase but retain the acronyms in uppercase using R? I need it for text mining and using udpi package. I could ofcourse use uppercase, but anyway to retain the uppercase acronyms while using lowercase?

tolower('NASA IS A US COMPANY').

tolower('NASA IS A US COMPANY')

tolower('NASA IS A US COMPANY')

Expected: NASA is a US company

Actual: nasa is a us company

Answer 1

We can do: test is the input:

paste(lapply(strsplit(test," "),function(x) ifelse(x %in% toupper(tm::stopwords()),
                                              tolower(x),x))[[1]],collapse=" ")
[1] "NASA is a US COMPANY"