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namespace boost{

template <class charT, class implementationT = sensible_default_choice>
struct regex_traits : public implementationT
   regex_traits() : implementationT() {}

template <class charT>
struct c_regex_traits;

template <class charT>
class cpp_regex_traits;

template <class charT>
class w32_regex_traits;

} // namespace boost

The class regex_traits is just a thin wrapper around an actual implementation class, which may be one of:

The default behavior can be altered by defining one of the following configuration macros in boost/regex/user.hpp

All these traits classes fulfil the traits class requirements.