Exception containing invalid utf-8

[BenjaminSchaaf]

When an invalid regex is provided, boost::regex throws an exception with an error message containing the regex pattern. When using a non-utf8 format the exception's std::string error message can end up containing invalid utf8.

The problem is on this line:

regex/include/boost/regex/v5/basic_regex_parser.hpp

Line 236 in ed6ebbd

message += std::string(m_base + start_pos, m_base + position);

If you use say boost::u32regex, then m_base is a char32_t *. Constructing a std::string from two char32_t * compiles but truncates characters, resulting in invalid utf8.

I'm not sure what the solution here is. In the mean time I've added a function to the character traits for converting a std::basic_string<charT> to std::string.