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

[Duncan Murdoch]

On 09/10/2023 7:57 p.m., Michael Chirico via R-devel wrote:
> 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.

I think currently we do as.character(str) (or some equivalent), so the 
test shouldn't require str to be a character to start.  For example, 
this is currently valid code:

   grepl(1, "abc123")

It's not great style, but shouldn't generate an error.

Duncan Murdoch

> 
> 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
> 
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