https://soooprmx.com/cocoa-objective-c-nsstring%ec%9d%98-%eb%ac%b8%ec%9e%90%ec%97%b4-%eb%92%a4%ec%a7%91%ea%b8%b0-%ea%b8%80%ec%9e%90%eb%a5%bc-%ec%a0%95%eb%a0%ac%ed%95%9c-%eb%ac%b8%ec%9e%90%ec%97%b4-%ea%b5%ac%ed%95%98/