a <- "text text NOTE 3/1"
b <- "text NOTE 4.3%"
gsub("(?:\\G(?!^)|NOTE)\\K\\s", "@", a, perl=TRUE)
# => [1] "text text NOTE@@@@@@3/1"
gsub("(?:\\G(?!^)|NOTE)\\K\\s", "@", b, perl=TRUE)
# => [1] "text NOTE@@@4.3%"
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