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)) ]
 
pat <- paste(regex.escape(sort.by.length.desc(v)), collapse="|")
pat <- paste0("(?s)", pat, "|(?:(?!", pat, ").)+")
pat
res <- regmatches(x, gregexpr(pat, x, perl=TRUE))
res

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