[Rd] FR: valid_regex() to test string validity as a regular expression

[Michael Chirico]

It will be useful to package authors trying to validate input which is
supposed to be a valid regular expression.

As near as I can tell, the only way we can do so now is to run any
regex function and check for the warning and/or condition to bubble
up:

valid_regex <- function(str) {
  stopifnot(is.character(str), length(str) == 1L)
  !inherits(tryCatch(grepl(str, ""), condition = identity), "condition")
}

That's pretty hefty/inscrutable for such a simple validation. I see a
variety of similar approaches in CRAN packages [1], all slightly
different. It would be good for R to expose a "canonical" way to run
this validation.

At root, the problem is that R does not expose the regex compilation
routines like 'tre_regcomp', so from the R side we have to resort to
hacky approaches.

Things get slightly complicated by encoding/useBytes modes
(tre_regwcomp, tre_regncomp, tre_regwncomp, tre_regcompb,
tre_regncompb; all in tre.h), but all are already present in other
regex routines, so this is doable.

Exposing a function to compile regular expressions is common in other
languages, e.g. Go [2], Python [3], JavaScript [4].

[1] https://github.com/search?q=lang%3AR+%2Fis%5Ba-zA-Z0-9._%5D*reg%5Ba-zA-Z0-9._%5D*ex.*%28%3C-%7C%3D%29%5Cs*function%2F+org%3Acran&type=code
[2] https://pkg.go.dev/regexp#Compile
[3] https://docs.python.org/3/library/re.html#re.compile
[4] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp