A regex component that attempts to transform a matched substring, saving the result if successful and backtracking if the transformation fails.

    struct TryCapture<Output>

    You use a TryCapture component to capture part of a match as a transformed value, when a failure to transform should mean the regex continues matching, backtracking from that point if necessary.

    The code below demonstrates using TryCapture to include a test that the Double value of a capture is over a limit. In the example, regex matches a dollar sign ("$") followed by one or more digits, a period ("."), and then two additional digits, as long as that pattern appears at the end of the line. The TryCapture block wraps the digits and period, capturing that part of the match separately and passing it to its transform closure. That closure converts the captured portion of the match, converts it to a Double, and only returns a non-nil value if it is over the transaction limit.

    let transactions = """
        CREDIT     109912311421    Payroll   $69.73
        CREDIT     105912031123    Travel   $121.54
        DEBIT      107733291022    Refund    $8.42
    let transactionLimit = 100.0
    let regex = Regex {
        TryCapture {
            Repeat(.digit, count: 2)
        } transform: { str -> Double? in
            let value = Double(str)!
            if value > transactionLimit {
                return value
            return nil

    When the TryCapture block’s transform closure processes the three different amounts in the list of transactions, it only returns a non-nil value for the $121.54 transaction. Even though the capture returns an optional Double value, the captured value is non-optional.

    // The type of each match's output is `(Substring, Double)`.
    for match in transactions.matches(of: regex) {
        print("Transaction amount: \(match.1)")
    // Prints "Transaction amount: 121.54"

    Throwing an error from a transform closure aborts matching and propagates the error out to the caller. If you want to capture the nil results of a failable transformation, instead of continuing a search, use Capture instead of TryCapture.

    Citizens in RegexBuilder