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Swift does not provide regex support in the language (hopefully in Swift 4!?). For now we still have to rely on NSRegularExpression.

Here is a String extension that extract the captured groups with a regex pattern.

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.matchesInString(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.rangeAtIndex(i)
            let matchedString = (self as NSString).substringWithRange(capturedGroupIndex)
            results.append(matchedString)
        }
        
        return results
    }
}

To use:

// Will match "bcde"
"abcdefg".capturedGroups(withRegex: "a(.*)f")

Capture Groups vs Matches

There is a difference.

The code regex.matchesInString(...) will return 0 or more matches. For example, “hello hello” will match the regex pattern “hello” twice.

We did not bother with multiple matches, and instead only handle for the first match.

Capture groups are regex pattern which you define that you want to capture using brackets eg. id=(\\d*) will capture a number after a “id=”.