https://soooprmx.com/swift-swift%ec%9d%98-string%ed%83%80%ec%9e%85-%ea%b8%b0%ec%b4%88-02-%eb%ac%b8%ec%9e%90%ec%97%b4-%ec%a1%b0%ec%9e%91/
(Swift) Swift의 String타입 기초 - 02. 문자열 조작