# Reference A reference to a captured portion of a regular expression. ``` struct Reference ``` ## Overview You can use a `Reference` to access a regular expression, both during the matching process and after a capture has been successful. In this example, the `kind` reference captures either `"CREDIT"` or `"DEBIT"` at the beginning of a line. Later in the regular expression, the presence of `kind` matches the same substring that was captured previously at the end of the line. ``` let kindRef = Reference(Substring.self) let kindRegex = ChoiceOf { "CREDIT" "DEBIT" } let transactionRegex = Regex { Anchor.startOfLine Capture(kindRegex, as: kindRef) OneOrMore(.anyNonNewline) kindRef Anchor.endOfLine } let validTransaction = "CREDIT 109912311421 Payroll $69.73 CREDIT" let invalidTransaction = "DEBIT 00522142123 Expense $5.17 CREDIT" print(validTransaction.contains(transactionRegex)) // Prints "true" print(invalidTransaction.contains(transactionRegex)) // Prints "false" ``` Any reference that is used for matching must be captured elsewhere in the `Regex` block. You can use a reference for matching before it is captured; in that case, the reference will not match until it has previously been captured. To access the captured “transaction kind”, you can use the `kind` reference to subscript a `Regex.Match` instance: ``` if let match = validTransaction.firstMatch(of: transactionRegex) { print(match[kindRef]) } // Prints "CREDIT" ``` To use a `Reference` to capture a transformed value, include a `transform` closure when capturing. ``` struct Transaction { var id: UInt64 } let transactionRef = Reference(Transaction.self) let transactionIDRegex = Regex { Capture(kindRegex, as: kindRef) OneOrMore(.whitespace) TryCapture(as: transactionRef) { OneOrMore(.digit) } transform: { str in UInt64(str).map(Transaction.init(id:)) } OneOrMore(.anyNonNewline) kindRef Anchor.endOfLine } if let match = validTransaction.firstMatch(of: transactionIDRegex) { print(match[transactionRef]) } // Prints "Transaction(id: 109912311421)" ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)