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 -- Philip Hazel -- ## List details at https://lists.exim.org/mailman/listinfo/pcre-dev
