Playing with regex in Swift...

String.captureGroups.swift

extension String {
    func capturedGroups(withRegex pattern: String) -> [String] {
        var results = [String]()
        
        var regex: NSRegularExpression
        do {
            regex = try NSRegularExpression(pattern: pattern, options: [])
        } catch {
            return results
        }
        
        let matches = regex.matches(in: self, options: [], range: NSRange(location:0, length: self.characters.count))
        
        guard let match = matches.first else { return results }
        
        let lastRangeIndex = match.numberOfRanges - 1
        guard lastRangeIndex >= 1 else { return results }
        
        for i in 1...lastRangeIndex {
            let capturedGroupIndex = match.rangeAt(i)
            let matchedString = (self as NSString).substring(with: capturedGroupIndex)
            results.append(matchedString)
        }
        
        return results
    }
}

let string: String = "Lorem ipsum **dolor** sit amet."
let boldGroups = string.capturedGroups(withRegex: "(\\*\\*([\\s\\S]+?)\\*\\*(?!\\*))")
let boldHTML = "<strong>\(boldGroups[1])</strong>"
let htmlBold = string.replacingOccurrences(of: boldGroups[0], with: boldHTML)