std::basic_string_view<CharT,Traits>::npos

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static constexpr size_type npos = size_type(-1);
(since C++17)

This is a special value equal to the maximum value representable by the type size_type. The exact meaning depends on context, but it is generally used either as end of view indicator by the functions that expect a view index or as the error indicator by the functions that return a view index.