Return the position of matching characters.

constexpr size_type
rfind(
    string_view str,
    size_type pos = string_view::npos) const;
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constexpr size_type
rfind(
    char c,
    size_type pos = string_view::npos) const;
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constexpr size_type
rfind(
    char const* s,
    size_type pos,
    size_type n) const;
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constexpr size_type
rfind(
    char const* s,
    size_type pos = string_view::npos) const;
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