вот пример:
> str <- "This is a string. In addition, this is a string!"
> str
[1] "This is a string. In addition, this is a string!"
> strsplit(gsub("[^[:alnum:] ]", "", str), " +")[[1]]
[1] "This" "is" "a" "string" "In" "addition" "this" "is" "a"
[10] "string"