소문자로 이루어진 문자열만 필터링하기 (CharacterSet)

import Foundation

func checkLowerCase(_ input: String) -> Bool {
    let lowercaseSet = CharacterSet.lowercaseLetters
    return input.rangeOfCharacter(from: lowercaseSet.inverted) == nil
}

let st = ["ss", "s1", "11", "SS", "d-"]

for s in st {
    if checkLowerCase(s) {
        print(s)
    }
}
// ss

https://developer.apple.com/documentation/foundation/characterset