Swift 5.1•Xcode 11或更高版本

extension Collection {
    var pairs: [SubSequence] {
        var startIndex = self.startIndex
        let count = self.count
        let n = count/2 + count % 2
        return (0..<n).map { _ in
            let endIndex = index(startIndex, offsetBy: 2, limitedBy: self.endIndex) ?? self.endIndex
            defer { startIndex = endIndex }
            return self[startIndex..<endIndex]
        }
    }
}

extension Collection {
    func distance(to index: Index) -> Int { distance(from: startIndex, to: index) }
}

extension StringProtocol where Self: RangeReplaceableCollection {
    mutating func insert<S: StringProtocol>(separator: S, every n: Int) {
        for index in indices.dropFirst().reversed()
            where distance(to: index).isMultiple(of: n) {
            insert(contentsOf: separator, at: index)
        }
    }
    func inserting<S: StringProtocol>(separator: S, every n: Int) -> Self {
        var string = self
        string.insert(separator: separator, every: n)
        return string
    }
}

测试中

let str = "112312451"

let final = str.pairs.joined(separator: ":")
print(final)      // "11:23:12:45:1"

let final2 = str.inserting(separator: ":", every: 2)
print(final2)      // "11:23:12:45:1\n"

var str2 = "112312451"
str2.insert(separator: ":", every: 2)
print(str2)   // "11:23:12:45:1\n"

var str3 = "112312451"
str3.insert(separator: ":", every: 3)
print(str3)   // "112:312:451\n"

var str4 = "112312451"
str4.insert(separator: ":", every: 4)
print(str4)   // "1123:1245:1\n"