std::wcscspn(3)

NAME
std::wcscspn - std::wcscspn

Synopsis
   Defined in header <cwchar>
   std::size_t wcscspn( const wchar_t* dest, const wchar_t* src );
   Returns the length of the maximum initial segment of the wide string pointed to by
   dest, that consists of only the characters not found in wide string pointed to by
   src.

Parameters
   dest - pointer to the null-terminated wide string to be analyzed
   src - pointer to the null-terminated wide string that contains the characters to
          search for

Return value
   The length of the maximum initial segment that contains only characters not found in
   the character string pointed to by src.

Example
   The output below was obtained using clang (libc++). // Run this code
 #include <cwchar>
 #include <iostream>
 #include <locale>
 int main()
 {
     wchar_t dest[] = L"白猫 黑狗 甲虫";
     // └───┐
     const wchar_t* src = L"甲虫,黑狗";
     const std::size_t len = std::wcscspn(dest, src);
     dest[len] = L'\0'; // terminates the segment to print it out
     std::wcout.imbue(std::locale("en_US.utf8"));
     std::wcout << L"The length of maximum initial segment is " << len << L".\n";
     std::wcout << L"The segment is \"" << dest << L"\".\n";
 }

Possible output:
 The length of maximum initial segment is 3.
 The segment is "白猫 ".

See also
           returns the length of the maximum initial segment that consists
   wcsspn of only the wide characters found in another wide string
           (function)
           finds the first location of any wide character in one wide string, in
   wcspbrk another wide string
           (function)
   C documentation for
   wcscspn