[Bug libstdc++/84110] Null character in regex throws std::regex_error

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84110

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:5df21c00aedb7878b8854901e95d7eda70266d31

commit r11-10118-g5df21c00aedb7878b8854901e95d7eda70266d31
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 29 13:48:11 2021 +0100

    libstdc++: std::basic_regex should treat '\0' as an ordinary char [PR84110]

    When the input sequence contains a _CharT(0) character, the strchr call
    in _Scanner<_CharT>::_M_scan_normal() will search for '\0' and so return
    a pointer to the terminating null at the end of the string. This makes
    the scanner think it's found a special character. Because it doesn't
    match any of the actual special characters, we fall off the end of the
    function (or assert in debug mode).

    We should check for a null character explicitly and either treat it as
    an ordinary character (for the ECMAScript grammar) or an error (for all
    others). I'm not 100% sure that's right, but it seems consistent with
    the POSIX RE rules where a '\0' means the end of the regex pattern or
    the end of the sequence being matched.

    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

    libstdc++-v3/ChangeLog:

            PR libstdc++/84110
            * include/bits/regex_error.h (regex_constants::_S_null): New
            error code for internal use.
            * include/bits/regex_scanner.tcc (_Scanner::_M_scan_normal()):
            Check for null character.
            * testsuite/28_regex/basic_regex/84110.cc: New test.

    (cherry picked from commit b701e1f8f6870c0f8cb4050674da489101dd05a5)