Method is_digit

include/stringzilla/stringzilla.hpp:2239–2239

    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()); }
    bool is_lower() const noexcept { return !empty() && contains_only(ascii_lowercase_set()); }
    bool is_space() const noexcept { return !empty() && contains_only(whitespaces_set()); }
    bool is_upper() const noexcept { return !empty() && contains_only(ascii_uppercase_set()); }