std::regex_traits<CharT>::lookup_classname (3) - Linux Manuals
template< class ForwardIt >
char_class_type lookup_classname( ForwardIt first,
bool icase = false ) const;
If the character sequence [first, last) represents the name of a valid character class in the currently imbued locale (that is, the string between [: and :] in regular expressions), returns the implementation-defined value representing this character class. Otherwise, returns zero.
If the parameter icase is true, the character class ignores character case, e.g. the regex [:lower:] with std::regex_constants::icase generates a call to regex_traits<>::lookup_classname() with [first, last) indicating the string "lower" and icase == true. This call returns the same bitmask as the call generated by the regex [:alpha:] with icase == false.
The following character classes are always recognized, in both narrow and wide character forms, and the classifications returned (with icase == false) correspond to the matching classifications obtained by the std::ctype facet of the imbued locale, as follows:
character class std::ctype classification
"w" std::ctype_base::alnum with '_' optionally added
The classification returned for the string "w" may be exactly the same as "alnum", in which case isctype() adds '_' explicitly.
Additional classifications such as "jdigit" or "jkanji" may be provided by system-supplied locales (in which case they are also accessible through std::wctype)
first, last - a pair of iterators which determines the sequence of characters that represents a name of a character class
icase - if true, ignores the upper/lower case distinction in the character classification
ForwardIt must meet the requirements of LegacyForwardIterator.
The bitmask representing the character classification determined by the given character class, or char_class_type() if the class is unknown.
demonstraits a custom regex_traits implementation of lookup_classname/isctype
// Run this code
isctype (public member function)