string_view support

`regex` doesn't appear to support `string_view`

```
{
std::string s;
boost::match_results m;
boost::regex_search(s, m, boost::regex(R"(\[quote.*?\])", boost::regex::icase), boost::match_default);
}
{
std::string_view sv;
boost::match_results m;
boost::regex_search(sv, m, boost::regex(R"(\[quote.*?\])", boost::regex::icase), boost::match_default);
// error: no matching function for call to 'regex_search(std::string_view&, boost::match_results&, boost::regex, boost::regex_constants::_match_flags)'
}
```

Start with the regex_search overloads involved in the shown std::string and std::string_view examples, and reproduce the compiler error. Done means the string_view example compiles and behaves like the std::string example.

Written by the indexing model from the issue text.