RFC: make std::regex unusable

[Benjamin De Kosnik]

> Can we make the constructor of std::basic_regex throw an exception, to
> stop people thinking it's supposed to work and then being surprised or
> annoyed that it's not implemented?

You could.
 
> For people who want to experiment with it anyway or help implement it
> we could suppress the exception if
> _GLIBCXX_YES_I_KNOW_REGEX_DOES_NOT_WORK is defined.
> 
> 
> Alternatively we could just stick a #warning in <regex>

... FYI there is a GSOC project working on <regex>. It might be better
to wait until that is done and re-assess.

-benjamin