String+isFirstLetters.swift

import Foundation

extension String {
    func isFirstLetters(count: Int, of characterSet: CharacterSet) -> Bool {
        
        guard characters.count >= count else {
            return false
        }
        
        let firstLetters = substring(to: index(startIndex, offsetBy: count))
        let isInSubset = CharacterSet(charactersIn: firstLetters).isSubset(of: characterSet
        
        )
        return isInSubset
    }
}

extension CharacterSet {
    static var vowels: CharacterSet {
        return CharacterSet(charactersIn: "AEIOUYaeiouy")
    }
}


"abcDEf".isFirstLetters(count: 3, of: .vowels) // false
"aioDEf".isFirstLetters(count: 3, of: .vowels) // true

"abcDEf".isFirstLetters(count: 1, of: .vowels) // true
"bbcDEf".isFirstLetters(count: 1, of: .vowels) // false
"ibcDEf".isFirstLetters(count: 1, of: .vowels) // true

"ABcdef".isFirstLetters(count: 3, of: .uppercaseLetters) // false
"ABDdef".isFirstLetters(count: 3, of: .uppercaseLetters) // true

"abBABcdef".isFirstLetters(count: 3, of: .lowercaseLetters) // false
"abbABDdef".isFirstLetters(count: 3, of: .lowercaseLetters) // true


"ABD".isFirstLetters(count: 3, of: CharacterSet(charactersIn: "A"..."C")) // false
"ABD".isFirstLetters(count: 3, of: CharacterSet(charactersIn: "A"..."D")) // true



// debug info

extension CharacterSet {
    var characters: [Character] {
        var result: [Character] = []
        for plane: UInt8 in 0...16 where self.hasMember(inPlane: plane) {
            for unicode in UInt32(plane) << 16 ..< UInt32(plane + 1) << 16 {
                if let uniChar = UnicodeScalar(unicode), self.contains(uniChar) {
                    result.append(Character(uniChar))
                }
            }
        }
        return result
    }
}



print(CharacterSet.uppercaseLetters.description) //<Foundation._SwiftNSCharacterSet: 0x6000000271a0>

print(String(CharacterSet.vowels.characters)) //AEIOUYaeiouy

print(CharacterSet.vowels.characters) // ["A", "E", "I", "O", "U", "Y", "a", "e", "i", "o", "u", "y"]

What I mean is you could do something like this:

func substringIsSubset(of characterSet: CharacterSet, from range: Range<String.Index>) -> Bool {
    guard range.lowerBound >= self.startIndex && range.upperBound <= self.endIndex else {
 	return false
     }
     return CharacterSet(charactersIn: substring(with: range)).isSubset(of: characterSet)
}