(In R) How to split words by title case in a string like "WeLiveInCA" into "We Live In CA" while preserving abbreviations?

Question

(In R) How to split words by title case in a string like "WeLiveInCA" into "We Live In CA" without splitting abbreviations?

I know how to split the string at every uppercase letter, but doing that would split initialisms/abbreviations, like CA or USSR or even U.S.A. and I need to preserve those.

So I'm thinking some type of logical like if a word in a string isn't an initialism then split the word with a space where a lowercase character is followed by an uppercase character.

My snippet of code below splits words with spaces by capital letters, but it breaks initialisms like CA becomes C A undesirably.

s <- "WeLiveInCA"
trimws(gsub('([[:upper:]])', ' \\1', s))
# "We Live In C A"

or another example...

s <- c("IDon'tEatKittensFYI", "YouKnowYourABCs")
trimws(gsub('([[:upper:]])', ' \\1', s))
# "I Don't Eat Kittens F Y I" "You Know Your A B Cs"

The results I'd want would be:

"We Live In CA"
#
"I Don't Eat Kittens FYI" "You Know Your ABCs"

But this needs to be widely applicable (not just for my example)

Answer 1

Try with base R gregexpr/regmatches.

s <- c("WeLiveInCA", "IDon'tEatKittensFYI", "YouKnowYourABCs")
regmatches(s, gregexpr('[[:upper:]]+[^[:upper:]]*', s))
#[[1]]
#[1] "We"   "Live" "In"   "CA"  
#
#[[2]]
#[1] "IDon't"  "Eat"     "Kittens" "FYI"    
#
#[[3]]
#[1] "You"  "Know" "Your" "ABCs"

Explanation.

1. [[:upper:]]+ matches one or more upper case letters;
2. [^[:upper:]]* matches zero or more occurrences of anything but upper case letters.
3. In sequence these two regular expressions match words starting with upper case letter(s) followed by something else.