---
title: Reference
framework: regexbuilder
role: symbol
role_heading: Structure
path: regexbuilder/reference
---

# Reference

A reference to a captured portion of a regular expression.

## Declaration

```swift
struct Reference<Capture>
```

## Overview

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)"

## Topics

### Initializers

- [init(_:)](regexbuilder/reference/init(_:).md)

## Relationships

### Conforms To

- [RegexComponent](swift/regexcomponent.md)

## See Also

### Captures

- [Capture](regexbuilder/capture.md)
- [TryCapture](regexbuilder/trycapture.md)