int iswalpha(wint_t c);
iswalpha returns true if the wide character c is alphabetic. Any character for which iswupper or iswlower returns true is considered alphabetic in addition to any of the locale-specific set of alphabetic characters for which none of iswcntrl, iswdigit, iswpunct, or iswspace is true.
In the ‘C’ locale, iswalpha returns nonzero (true) if and only if iswupper or iswlower return true for the value of the argument c.