Function is_name_start

extlibs/fmt/include/fmt/format.h:1980–1982

};

template <typename Char> FMT_CONSTEXPR bool is_name_start(Char c) {
  return ('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z') || '_' == c;
}

// Parses the range [begin, end) as an unsigned integer. This function assumes
// that the range is non-empty and the first character is a digit.