cpp/regex/regex traits/lookup classname

If the character sequence 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 is, the character class ignores character case, e.g. the regex  with std generates a call to  with  indicating the string  and. This call returns the same bitmask as the call generated by the regex with.

The following character classes are always recognized, in both narrow and wide character forms, and the classifications returned (with ) correspond to the matching classifications obtained by the std facet of the imbued locale, as follows:

The classification returned for the string may be exactly the same as, in which case isctype adds  explicitly.

Additional classifications such as or  may be provided by system-supplied locales (in which case they are also accessible through std).

Return value
The bitmask representing the character classification determined by the given character class, or if the class is unknown.