split string without loss of characters

Question

I wish to split strings at a certain character while retaining that character in the second resulting string. I can achieve almost all of the desired operation, except that I lose the characters I specify in strsplit, which I guess is called the delimiter.

Is there a way to request that strsplit retain the delimiter? Or must I use a regular expression of some kind? Thank you for any advice. This seems like a very basic question. Sorry if it is a duplicate. I prefer to use base R.

Here is an example showing what I have so far:

my.table <- read.table(text = '
                                                            model npar     AICc 
 AA(~region+state+county+city)BB(~region+state+county+city)CC(~1)   17 11111.11
         AA(~region+state+county)BB(~region+state+county)CC(~123)   14 22222.22
                        AA(~region+state)BB(~region+state)CC(~33)   13 33333.33
                                  AA(~region)BB(~region)CC(~4321)    6 44444.44
', header = TRUE, stringsAsFactors = FALSE)

desired.result <- read.table(text = '
                                                      model        CC npar     AICc
 AA(~region+state+county+city)BB(~region+state+county+city)    CC(~1)   17 11111.11
           AA(~region+state+county)BB(~region+state+county)  CC(~123)   14 22222.22
                         AA(~region+state)BB(~region+state)   CC(~33)   13 33333.33
                                     AA(~region)BB(~region) CC(~4321)    6 44444.44
', header = TRUE, stringsAsFactors = FALSE)

split.model  <- strsplit(my.table$model, 'CC\\(')

split.models <- matrix(unlist(split.model), ncol=2, byrow=TRUE, dimnames = list(NULL, c("model", "CC")))

desires.result2 <- data.frame(split.models, my.table[,2:ncol(my.table)])
desires.result2

#                                                       model     CC npar     AICc
# 1 AA(~region+state+county+city)BB(~region+state+county+city)    ~1)   17 11111.11
# 2           AA(~region+state+county)BB(~region+state+county)  ~123)   14 22222.22
# 3                         AA(~region+state)BB(~region+state)   ~33)   13 33333.33
# 4                                     AA(~region)BB(~region) ~4321)    6 44444.44

Answer 1

The basic idea is to use look-around operations from regular expressions to strsplit to get your desired result. However, it's a bit trickier than that with strsplit and positive lookahead. Read this excellent post from @JoshO'Brien for explanation.

pattern <- "(?<=\\))(?=CC)"
strsplit(my.table$model, pattern, perl=TRUE)
# [[1]]
# [1] "AA(~region+state+county+city)BB(~region+state+county+city)"
# [2] "CC(~1)"                                                    

# [[2]]
# [1] "AA(~region+state+county)BB(~region+state+county)"
# [2] "CC(~123)"                                        

# [[3]]
# [1] "AA(~region+state)BB(~region+state)" "CC(~33)"                           

# [[4]]
# [1] "AA(~region)BB(~region)" "CC(~4321)"             

Of course, I leave the task of do.call(rbind, ...) and cbind to get the final desired.output to you.

Answer 2

Almost right after I posted I thought of using gsub to insert a space and then split on the space. Although, I like Arun's answer better.

my.table <- read.table(text = '
                                                            model npar     AICc 
 AA(~region+state+county+city)BB(~region+state+county+city)CC(~1)   17 11111.11
         AA(~region+state+county)BB(~region+state+county)CC(~123)   14 22222.22
                        AA(~region+state)BB(~region+state)CC(~33)   13 33333.33
                                  AA(~region)BB(~region)CC(~4321)    6 44444.44
', header = TRUE, stringsAsFactors = FALSE)

my.table$model <- gsub("CC", " CC", my.table$model)

split.model <- strsplit(my.table$model, ' ')

split.models <- matrix(unlist(split.model), ncol=2, byrow=TRUE, dimnames = list(NULL, c("model", "CC")))

desires.result <- data.frame(split.models, my.table[,2:ncol(my.table)])
desires.result

#                                                        model        CC npar     AICc
# 1 AA(~region+state+county+city)BB(~region+state+county+city)    CC(~1)   17 11111.11
# 2           AA(~region+state+county)BB(~region+state+county)  CC(~123)   14 22222.22
# 3                         AA(~region+state)BB(~region+state)   CC(~33)   13 33333.33
# 4                                     AA(~region)BB(~region) CC(~4321)    6 44444.44

Answer 3

... why not just tack the separator back on afterwards? Would seem to save a lot of trouble fiddling with regexes.

split.model <- lapply(strsplit(my.table$model, 'CC\\('), function(x) {
    x[2] <- paste0("CC(", x[2])
    x
})