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

From cppreference.com
static constexpr size_type npos = size_type ( - 1 ) ;
(library fundamentals TS)

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.