Return the position of the first occurrence of a character not belonging to the specified characterString, if such an occurrence can be found in this string (on or after the optionally specified position if such a position is specified), and return npos otherwise.
Declared in <bslstl_string.h>
size_type
find_first_not_of(
basic_string<char8_t> const& characterString,
size_type position = 0) const noexcept;