Using Regex in Swift

In this article I will be walking you through the various different ways to define and use regular expressions in Swift.

Throughout this article I will be using the following string as the sample text we are trying to parse with Regex. It is an example of a basic CSV table that one might encounter in their day-to-day work. I stole this example text from here because it's a very good example that shows the flexibility of Regex in Swift.

let input = """
CREDIT    03/02/2022    Payroll                   $200.23
CREDIT    03/03/2022    Sanctioned Individual A   $2,000,000.00
DEBIT     03/03/2022    Totally Legit Shell Corp  $2,000,000.00
DEBIT     03/05/2022    Beanie Babies Forever     $57.33
"""

Let's get started.

`NSRegularExpression`

If your project is targeting platforms before iOS 16, then you'll have no choice but to use Foundation's Regex APIs. These are available since iOS 4.0 and were originally made for Objective-C. As a result they're a little tedious to use in Swift.

If we wanted to define a regex pattern to grab the value of first column in the table, we would do it like so.

let regex = try NSRegularExpression(pattern: "(CREDIT|DEBIT)\\s+")

This gives us an NSRegularExpression object. We have to try the initializer because if the string that we pass in happens to be invalid Regex it will throw an error.

Notice the double back slash in \\s+. We can avoid having to escape the backslash by using extended string delimiters. A special feature in swift that allows us to use a more specific string delimiter rather than just double quotes. With this, our regex pattern looks as follows:

let regex = try NSRegularExpression(pattern: #"(CREDIT|DEBIT)\s+"#)

To actually use the regex on our string, we have to call methods on the regex object such as firstMatch(in:options:range:), matches(in:options:range:) or enumerateMatches(in:options:range:using:). For example

regex.enumerateMatches(
  in: input,
  range: NSRangeFromString(input)
) { result, flags, stopPointer in
  // Do something with result

  // Stop early if needed
  let shouldStop = false
  if shouldStop {
    stopPointer.pointee = false
  }
}

This API is a little bulky and dated in my opinion. I wont spend much more time exploring it. If you want to find out more, check out the docs.

`Regex`

Starting with iOS 16.0 Swift has a new Regex type as part of its standard library. You can use this type much like you used the NSRegularExpression type above to define a regex pattern with a string literal.

let regex = try Regex(#"(CREDIT|DEBIT)\s+"#)

The usage of our regex type is a little different. The Regex object exposes a few method that you can use to parse a string, however the bulk of the API is defined on String. If we want to get the first match on our input string, we would do it like so.

let match = input.firstMatch(of: regex)

This returns a Regex.Match object that we can use to query the output of our regex pattern. You can then use match.output to get the output of the regex.

So far this is not much different than what we had before. We still have to provide a string literal, and we still have to try the initializer, as it might fail if the regex is invalid. However this new API has some hidden improvements that we haven't made use of yet.

Output types

Regex is generic over it's output type. What this means, is that when we define capture groups in our regex, we can actually specify them on the Regex as the output of the object. To do this, you provide an extra as: argument to the initializer, where you specify a tuple type of what you're expecting. Most of the time this will be a tuple of two or more Substrings.

let regex = try Regex(#"(CREDIT|DEBIT)\s+"#, as: (Substring, Substring).self)

Notice that I specified a tuple of two Substrings even though I only have one capture group. That is because the first captured of the regex is always the whole string that was matched. Which is then followed by all of the explicit capture groups.

If I wanted to also capture the first number of the date in the following column, I would use a regex like this

let regex = try Regex(#"(CREDIT|DEBIT)\s+(\+d)"#, as: (Substring, Substring, Substring).self)

Do get that number out of the match, I would simply access the tuple element on the match object like so.

let match = input.firstMatch(of: regex)
let month = match?.2  // "03"

The type of month will be Substring?. I can similarly get the first column by accessing match?.1, or the whole matched string with match?.0.

If I wanted to get a list of all the values in the first column I could do something like ths

let transactionTypes = input.matches(of: regex).map(\.1)  // ["CREDIT", "CREDIT", "DEBIT", "DEBIT"]

Hopefully you can see how this is an improvement over Foundation's NSRegularExpression, however it doesn't stop there.

Regex Literals

With SE-0354 the Swift team introduced a new syntax for defining regex patterns called Regex Literals. Let's take a look at it in this section.

If I wanted to define the exact same pattern that we had in the previous example, using regex literals it would look as follows

let regex = /(CREDIT|DEBIT)\s+(\d+)/

As you can see, it is literally the raw regex pattern surrounded by forward slashes (/). One benefit of defining a regex patterns this way rather than creating the raw Regex type is that these literals are checked at compile time. This means if you make an error, you will see it right away, so you don't need any trys. It also means that the Swift compiler can analyze your pattern and figure out the correct return type for the regex.

If we look at the type of regex with print(type(of: regex)) we would see Regex<(Substring, Substring, Substring)>. As you can see, the compiler was able to automatically determine the type of the pattern, without us having to specify it manually. You can also see that the type that regex literals create is still a Regex, so its usage is exactly the same as in the previous example.

Extended Literal Delimiters

There is one gotcha that I should mention, and that is the fact that forward slashes must be escaped when using regex literals. Let me show you what I mean. Below is a sample of what a row in our data looks like, in case you forgot.

CREDIT    03/02/2022    Payroll                   $200.23

If we wanted to grab the entire date from the second column, the regex pattern for that would look like so

let regex = /(CREDIT|DEBIT)\s+(\d+)/(\d+)/(\d+)/

However this would not work. Notice the forward slashes in (\d+)/(\d+)/(\d+). Because regex literal are delimited by forward slashes, the compiler is likely to be yelling all kinds of errors at you right now. To make this work, we have to escape the forward slashes (/) with backslashes (\). So our escaped regex looks like this

let regex = /(CREDIT|DEBIT)\s+(\d+)\/(\d+)\/(\d+)/

This is a little difficult to work with in my opinion. Which is why the Swift team decided to enable the same kind of extended delimiters that they made for string literals, but for regex literals. So to avoid the backslash escaping we can do

let regex = #/(CREDIT|DEBIT)\s+(\d+)/(\d+)/(\d+)/#

One hidden feature of using these regex delimiters is that all whitespace and newlines in the regex are ignored. This means that you can break it up across multiple lines to make it more readable. Not only that but you can actually leave comments within your regex with by starting it with #.

let regex = #/
  (CREDIT|DEBIT)     # transaction type
  \s+
  (\d+)/(\d+)/(\d+)  # date
/#

I find this to be way more readable than the string regex patterns that we were using before.

Of course as mentioned before, this produces a Regex type, so usage is the same as before.

let match = input.firstMatch(of: regex)
let month = match?.2  // "03"

Named Capture Groups

Notice how we have to use numbers to refer to our capture groups. Another benefit of using regex literals is allowing you to name the capture groups. This is done by adding a ? after the opening parenthesis and providing the name between angle brackets right after that.

let regex = #/
  (?<transactionType> CREDIT|DEBIT)          # transaction type
  \s+
  (?<month> \d+)/(?<day> \d+)/(?<year> \d+)  # date
/#

Now, when we access the capture groups we can use the names we specify in the regex, rather than the number of their position.

let match = input.firstMatch(of: regex)
let month = match?.month  // "03"
let year = match?.year    // "02"
let day = match?.day      // "2022"

This is definitely an improvement over what Foundation offered. But wait, there's more!

`RegexBuilder`

Along with regex literals, the Swift team introduced the RegexBuilder framework. This is a framework that let's us define regex patterns using Swift's result builder syntax, similar to how we define views in SwiftUI.

Let's define the same pattern we were working with, using RegexBuilder this time.

First we import the framework.

import RegexBuilder

Next we start a pattern definition. We will define our pattern within the curly braces.

let regex = Regex {

}

Let's look at the first part of the regex.

(CREDIT|DEBIT)

This says, look for either the whole string `CREDIT` or the whole string `DEBIT` and then capture that. To achieve the same thing with RegexBuilder, we quite literally just say what we want.

let regex = Regex {
  Capture {
    ChoiceOf {
      "CREDIT"
      "DEBIT"
    }
  }
}

Notice how much more descriptive this approach is. Rather than using the cryptic regex syntax, we just use normal english words to describe what we're trying to do. When you're coming back six months from now, you won't need to pull out a regex handbook to understand what you're trying to do here.

In fact I bet that you can understand the rest of the pattern without me explaining much of it.

let regex = Regex {
  // Transaction type
  Capture {
    ChoiceOf {
      "CREDIT"
      "DEBIT"
    }
  }

  OneOrMore {
    CharacterClass.whitespace
  }

  // Date
  Capture {
    OneOrMore {
      CharacterClass.digit
    }
  }
  "/"
  Capture {
    OneOrMore {
      CharacterClass.digit
    }
  }
  "/"
  Capture {
    OneOrMore {
      CharacterClass.digit
    }
  }
}

There are a few things to notice here. First, since this is just plain Swift code, we can leave inline comments, like we would anywhere else.

Second, we use common constructs like Capture, OneOrMore and ChoiceOf, that if you're familiar with you know right away what they do, and if not, then it's easy to guess based on the name. It's intuitive.

Third, since this is using Swift's result builders, we can compose different regex patterns together, similar to how you'd extract repetitive View code in SwiftUI.

let digitCapture = Regex {
  Capture {
    OneOrMore {
      CharacterClass.digit
    }
  }
}

let regex = Regex {
  // Transaction type
  Capture {
    ChoiceOf {
      "CREDIT"
      "DEBIT"
    }
  }

  OneOrMore {
    CharacterClass.whitespace
  }

  // Date
  digitCapture
  "/"
  digitCapture
  "/"
  digitCapture
}

In terms of usage this is a still that same Regex type. So we use regex in exactly the same way as we did before. Except since we haven't named our captures yet, we must use the tuple indices.

let match = input.firstMatch(of: regex)
let month = match?.2  // "03"

The string API exposes an additional set of methods that allow us to pass in regex builders directly. This is particularly useful when your regex is short.

let match = input.firstMatch {
  Capture {
    ChoiceOf {
      "CREDIT"
      "DEBIT"
    }
  }
}
let transactionType = match?.1

Naming Capture Groups

When it comes to naming capture groups, this works a little different from the other examples. First, we must define a Reference that will act as the gluing component between defining the capture group, and actually getting the values out.

Let's define a reference for the transaction type group.

let transactionTypeRef = Reference(Substring.self)

When defining a reference, you must specify what kind of data it is capturing. For most purposes this will be Substring, however there is a way to use a custom type. More on that later.

We then use the reference in our regex to associate a particular Capture with this reference. We do this by supplying the reference as an argument to the Capture expression.

Capture(as: transactionTypeRef) {
  ChoiceOf {
    "CREDIT"
    "DEBIT"
  }
}

If we try to access the transaction type from a match using the tuple index, it will not work. Neither will match.transactionType. Unfortunately that is not possible. Instead, we have to access data on the match object as if it's a dictionary.

let match = input.firstMatch(of: regex)
let transactionType = match?[transactionTypeRef]  // "CREDIT"

This returns an optional Substring, just like the output type of our transactionTypeRef.

We can repeat this process for the date fields, turning the extrapolated digitCapture property into a function. Here's what that would look like.

let transactionTypeRef = Reference(Substring.self)
let dayRef = Reference(Substring.self)
let monthRef = Reference(Substring.self)
let yearRef = Reference(Substring.self)

func digitCapture(as ref: Reference<Substring>) -> some RegexComponent {
  Capture(as: ref) {
    OneOrMore {
      CharacterClass.digit
    }
  }
}

let regex = Regex {
  Capture(as: transactionTypeRef) {
    ChoiceOf {
      "CREDIT"
      "DEBIT"
    }
  }
  OneOrMore {
    CharacterClass.whitespace
  }
  digitCapture(as: monthRef)
  "/"
  digitCapture(as: dayRef)
  "/"
  digitCapture(as: yearRef)
}

Notice here that we specified a return type of some RegexComponent for the digitCapture function. This again is very similar to how SwiftUI works, where we specify a return type of some View for custom views. We could of course use a concrete Regex type, but then we would need to figure out a concrete return type from our builder expression anytime the pattern changes, and unfortunately it's not as simple as Regex<(Substring, Substring)>.

Notice also that we didn't need to wrap our digit capture pattern in a Regex { ... } block. That is because Capture along with most other Regex Builder blocks all conform to RegexComponent. This can save you some code when composing regex patterns together.

With this code in place, we should be able to read all the captured fields in the regex.

let match = input.firstMatch(of: regex)
let transactionType = match?[transactionTypeRef]  // "CREDIT"
let year = match?[yearRef]                        // "2022"
let day = match?[dayRef]                          // "02"
let month = match?[monthRef]                      // "03"

Capturing Custom Types

The last thing that I want to mention has to do with string parsing. We can use these regex patterns to fish out parts of strings, but this just gives us Substrings. There is a way to have the regex pattern transform those Substrings into custom data types for us. Let's look at how we can do that.

Let's assume that we have a custom enum to represent the different kinds of transactions that we can encounter.

enum TransactionType: String {
  case credit = "CREDIT"
  case debit = "DEBIT"
}

We can specify a transformation function in our regex, which will take the captured CREDIT or DEBIT strings and automatically turn them into our TransactionType.credit or .debit instances.

To do this, we change our CREDIT/DEBIT capture regex to something like this

TryCapture(as: transactionTypeRef) {
  ChoiceOf {
    "CREDIT"
    "DEBIT"
  }
} transform: { substring in
  TransactionType(rawValue: String(substring))
}

The way this works is the TryCapture will take in whatever it matches within its first { ... } block, and pass that to the transform function as the first argument. The type of this argument will be Substring. From there you simply parse it into whatever custom type you want and return that. The return type of the transform function is optional so you can return nil if the substring doesn't match. You can even throw errors in there.

One last change still remains, and that is to change the output type of our reference. Replace Substring.self with TransactionType.self in the definition of transactionTypeRef.

let transactionTypeRef = Reference(TransactionType.self)  // TransactionType.credit

Now when you access the transaction type from a match object, rather than returning a Substring instance, you should get a TransactionType instance.

Conclusion

This was an overview of the changes and improvements introduced to working with regex in Swift. I hope you find this article useful.

Sources

• NSRegularExpression

• Extended String Delimiters

• SE-0350 - Regex Type Overview

• SE-0351 - Regex Builder

• SE-0354 - Regex Literals

• Regex

• RegexBuilder