https://stackoverflow.com/a/33306676/1271826

SplitedString.swift

// modern syntax of `NSRegularExpression`

func splitedString(string: String, length: Int) -> [String] {
    let regexString = "(\\d{1,\(length)})"
    let nsstring = string as NSString
        
    let regex = try! NSRegularExpression(pattern: regexString)
    return regex
        .matches(in: string, range: NSRange(string.startIndex..., in: string))
        .map { nsstring.substring(with: $0.range) }
}

// or to eliminate NS types

func splitedString(string: String, length: Int) -> [Substring] {
    let regexString = "(\\d{1,\(length)})"
    var substrings: [Substring] = []

    var searchStart: String.Index? = string.startIndex

    repeat {
        if let range = string.range(of: regexString, options: .regularExpression, range: searchStart!..<string.endIndex) {
            substrings.append(string[range])
            searchStart = range.upperBound
        } else {
            searchStart = nil
        }
        
    } while searchStart != nil
    
    return substrings
}

// or, to use the new `Regex` type:

import RegexBuilder

func splitedStringWithNewRegex(string: String, length: Int) -> [Substring] {
    let regex = Regex { Repeat(.digit, 1...length) }
    return string.matches(of: regex).map {
        $0.0
    }
}