UnderlineUnicodeStringExtension.swift

extension String {
    
    init<S: Sequence>(unicodeScalars ucs: S) where S.Iterator.Element == UnicodeScalar {
        var s = ""
        s.unicodeScalars.append(contentsOf: ucs)
        self = s
    }
    
    var underlineUnicode: String? {
        let underlineUnicode: UInt32 = 818
        guard let underlineUnicodeScalar = Unicode.Scalar(underlineUnicode) else { return nil }
        var resultString = ""
        for unicodeScalar in self.unicodeScalars {
            guard let characterUnicodeScalar = Unicode.Scalar(unicodeScalar.value) else { return nil }
            let underlineCharacter = String(unicodeScalars: [characterUnicodeScalar, underlineUnicodeScalar])
            resultString.append(underlineCharacter)
        }
        return resultString
    }
}

print("test".underlineUnicode)
// Optional("t̲e̲s̲t̲")