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

From: Edward Diener (eldiener_at_[hidden])
Date: 2020-04-07 19:33:58

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 ?