Hi,
Thanks for the detailed reply, that's very useful. To be honest, I
won't work on implementing this myself, but it's important to know
what's possible to implement when designing APIs.
I think it would be OK for Julia to check whether a string is valid
UTF-8 beforehand (as PCRE currently does), and fall back to a slow path
if it's not. Of course the slow path shouldn't make the standard path
slower, and ideally code duplication would be limited, which might not
be easy. Or maybe the string could be made valid before passing it to
PCRE, replacing invalid sequences with special characters which could
then be reintroduced in the matches in the corresponding positions.
For now I guess we should require strings to be valid, and if somebody
is able to implement this later we can always remove this requirement,
as it wouldn't be breaking.
Thanks for your help
Le vendredi 13 avril 2018 à 16:07 +0100, p...@hermes.cam.ac.uk a
écrit :
> On Thu, 12 Apr 2018, Milan Bouchet-Valat wrote:
> 
> > I'm writing on behalf of the Julia programming language [1]
> > developers
> > in order to get some information regarding the handling of invalid
> > UTF-
> > 8 string when PCRE2_UTF and PCRE2_NO_UTF_CHECK flags are set. 
> 
> Milan,
> 
> I understand what you are suggesting (treating invalid UTF-8 as one-
> byte 
> characters) because I have implemented exactly that in other
> software 
> I've written where performance is not critical.
> 
> However, in regex matching, performance *is* critical, which is why
> PCRE 
> insists on working only with valid UTF strings. Checking each
> sequence 
> for validity each time a character was inspected would degrade 
> performance. (Also, in a backtracking algorithm, the same character
> may
> be inspected multiple times during the course of a match, which only 
> makes matters worse.)
> 
> The code in the PCRE2 library that checks a UTF-8 string for validity
> is
> non-trivial. (It's in the source file src/pcre2_valid_utf.c if you
> want
> to take a look.) Admittedly, it does identify very specific errors in
> invalid sequences, but, for example, checking a 3-byte sequence
> involves
> seven "if" tests of various kinds plus a switch and a table lookup.
> (That's from a quick visual scan of the code; hope I counted right.)
> Ignoring some of the less serious errors (overlong sequences or
> surrogate codes) would simplify this a bit, but not much.
> 
> My view on this has always been that the most efficient approach, in
> the 
> sense of getting the "best" (in some sense) behaviour over all
> applications, is for applications to handle non-standard character
> strings external to PCRE so that it can work as efficiently as
> possible.
> One possible approach for strings of unknown provenance is to run
> without PCRE2_NO_UTF_CHECK and, if any of the "invalid UTF" errors
> occur, to convert the string (according to whatever rules you want)
> into
> a valid UTF-8 string and then try again.
> 
> > Do you think such a behavior would make sense? Could it be
> > implemented
> > without dramatically impacting performance? Julia could use a
> > custom
> > patch if this feature is not deemed useful for PCRE.
> 
> It certainly makes sense, but I don't think it could be implemented 
> without a serious performance hit. If you want to hack and try, note 
> that the macros whose names start with GETCHAR (in
> pcre2_intmodedep.h) 
> are used for character handling. In the case of UTF-8 these make use
> of 
> GETUTF8, GETUTF8INC, and GETUTF8LEN, which are defined in 
> pcre2_internal.h. However, there are also BACKCHAR, FORWARDCHAR, and 
> ACROSSCHAR for moving around. These macros are used for compilation
> as 
> well as for matching by the interpreter functions pcre2_match() and 
> pcre2_dfa_match(). I don't know what happens in the JIT matcher, as I
> do 
> not maintain that code, but it too assumes valid UTF-8. To be honest,
> I 
> don't really advise trying to hack in this way. I think it makes
> more 
> sense to fix bad strings externally.
> 
> Philip
> 
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