x <- "abc[[+de.f[-[[g"
v <- c("+", "-", "[", "[[")
 
## Escaping function
regex.escape <- function(string) {
  gsub("([][{}()+*^$|\\\\?])", "\\\\\\1", string)
}
## Sorting by length in the descending order function
sort.by.length.desc <- function (v) v[order( -nchar(v)) ]
## Interleaving function
riffle3 <- function(a, b) { 
  mlab <- min(length(a), length(b)) 
  seqmlab <- seq(length=mlab) 
  c(rbind(a[seqmlab], b[seqmlab]), a[-seqmlab], b[-seqmlab]) 
} 
pat <- paste(regex.escape(sort.by.length.desc(v)), collapse="|")
res <- riffle3(regmatches(x, gregexpr(pat, x), invert=TRUE)[[1]], regmatches(x, gregexpr(pat, x))[[1]])
res <- res[res != ""]
res

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