Invalid Perl regex leads to misaligned access

[cmazakas]

The following code:

int main()
{
   // char const strdata[] = "\x00t\x03.z%(?x:]*+\x0c#\\x0c\x0c\x0c+\x0c#\\x0c\x0c\x0c\x11\x0c\x0c\xff\xff\xfd*\xff\xff\xff\xff\xff\xff\xff\xff|\xff\xff\xfd*\xff\xff)*\x01\x03\x00\x00\x00\x03\xff\xff\xff\x00\x00\xff\xff\xff";
   char const strdata[] = "(?x:]*+#\x0c+)*";
   
   std::string str(strdata, strdata + sizeof(strdata) - 1);
   boost::regex e(str);
   std::string text = "hello, world!";

   boost::match_results<std::string::const_iterator> what;
   bool match = boost::regex_match(text, what, e,
                      boost::match_default | boost::match_partial);
   (void)match;
}

will cause the following runtime failure with ubsan:

boost/regex/v5/basic_regex_creator.hpp:717:21: runtime error: member access within misaligned address 0x51d0000001b7 for type 're_syntax_base', which requires 8 byte alignment
0x51d0000001b7: note: pointer points here
 00 00 01 be be  00 00 00 00 00 00 00 00  b7 01 00 00 d0 51 00 00  00 00 00 00 00 01 be be  00 00 00

We erroneously accept this regex. Perl fails with:

Unmatched ( in regex; marked by <-- HERE in m/( <-- HERE ?x:]*+#\x0c+)*/ at test.pl line 3.

when given:

"rawr" =~ /(?x:]*+#\x0c+)*/

[cmazakas]

This manifests in CI here: https://github.com/cmazakas/regex/actions/runs/13401006696/job/37431641879#step:15:387