Method is_alpha

include/stringzilla/stringzilla.hpp:3241–3241

    bool contains_only(byteset set) const noexcept { return find_first_not_of(set) == npos; }
    bool is_alpha() const noexcept { return !empty() && contains_only(ascii_letters_set()); }
    bool is_alnum() const noexcept { return !empty() && contains_only(ascii_letters_set() | digits_set()); }
    bool is_ascii() const noexcept { return empty() || contains_only(ascii_controls_set() | ascii_printables_set()); }
    bool is_digit() const noexcept { return !empty() && contains_only(digits_set()); }