[boost] [xpressive] Where are the names in the c_ctype.h file coming from

In the xpressive header file c_ctype.h there are names such as _ALPHA, _UPPER, _LOWER, _DIGIT etc. Where are these names declared or defined ? I searched through all the Boost libraries and did not find any of these names declared or defined anywhere. Are they supposed to be part of some C++ standard header file ?