[Rd] Bug in perl=TRUE regexp matching?

[Brodie Gaslam]

On 7/23/23 4:29 PM, Duncan Murdoch wrote:
> The help page for `?gsub` says (in the context of performance 
> considerations):
> 
> 
> "... just one UTF-8 string will force all the matching to be done in 
> Unicode"

It's been a little while since I looked at the code but IIRC this just 
means that strings are converted to UTF-8 before matching.  The problem 
here seems to be more about the interpretation of the "\\w+" token by 
PCRE.  I think this makes it a little clearer what's going on:

     gsub("\\w", "a", "Γ", perl=TRUE)
     [1] "Γ"

So no match.  The PCRE docs 
https://www.pcre.org/original/doc/html/pcrepattern.html (this might be 
the old docs, but it works for our purposes here) mention we can turn on 
unicode property matching with the "(*UCP)" token:

      gsub("(*UCP)\\w", "a", "Γ", perl=TRUE)
      [1] "a"

So there are two layers at play here.  The first one is whether R 
converts strings to UTF-8, which I think is what the documentation is 
about.  The other is whether the PCRE engine is configured to recognize 
Unicode properties, which at least in both of our configurations for 
this specific case it appears like it is not.

Best,

B.


> 
> 
> However, this thread on SO:  https://stackoverflow.com/q/76749529 gives 
> some indication that this is not true for `perl = TRUE`.  Specifically:
> 
>  > strings <- c("89 562", "John Smith", "Γιάννης Παπαδόπουλος", 
> "Jean-François Dupuis")
>  > Encoding(strings)
> [1] "unknown" "unknown" "UTF-8"   "UTF-8"
>  > regex <- "\\B\\w+| +"
>  > gsub(regex, "", strings)
> [1] "85"   "JS"   "ΓΠ"   "J-FD"
> 
>  > gsub(regex, "", strings, perl = TRUE)
> [1] "85"                  "JS"                  "ΓιάννηςΠαπαδόπουλος" 
> "J-FçoD"
> 
> and the website https://regex101.com/r/QDFrOE/1 gives the first answer 
> when the regex option /u ("match with full Unicode) is specified, but 
> the second answer when it is not.
> 
> Now I'm not at all sure that that website is authoritative, but this 
> looks like a flag may have been missed in the `perl = TRUE` case.
> 
> Duncan Murdoch
> 
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