[Rd] Bug in PCRE interface code

[Duncan Murdoch]

This Stackoverflow question https://stackoverflow.com/q/77036362 turned 
up a bug in the R PCRE interface.

The example (currently in an edit to the original question) tried to use 
named capture with more than 127 named groups.  Here's the code:

append_unique_id <- function(x) {
   for (i in seq_along(x)) {
     x[i] <- paste0("<", paste(sample(letters, 10), collapse = ""), ">", 
x[i])
   }
   x
}

list_regexes <- sample(letters, 128, TRUE) # <<<<<<<<<<< change this to
                                            #             127 and it works
regex2 <- append_unique_id(list_regexes)
regex2 <- paste0("(?", regex2, ")")
regex2 <- paste(regex2, collapse = "|")

out <- gregexpr(regex2, "Cyprus", perl = TRUE, ignore.case = TRUE)
#> Error in gregexpr(regex2, "Cyprus", perl = TRUE, ignore.case = TRUE): 
attempt to set index -129/128 in SET_STRING_ELT

I think the bug is in R, here: 
https://github.com/wch/r-source/blob/57d15d68235dd9bcfaa51fce83aaa71163a020e1/src/main/grep.c#L3079

This is the line

	    int capture_num = (entry[0]<<8) + entry[1] - 1;

where entry is declared as a pointer to a char.  What this is doing is 
extracting a 16 bit number from the first two bytes of a character 
string holding the name of the capture group.  Since char is a signed 
type, the conversion of bytes to integer gets messed up and the value 
comes out wrong.

Duncan Murdoch