Fast and powerful Swift string scanning made simple

What's New

Sweep 0.4.0


A single substring can now easily be scanned for using the new firstSubstring(between:and:) API.


Swift Package Manager Mac + Linux Twitter: @johnsundell

Welcome to Sweep — a powerful and fast, yet easy to use, Swift string scanning library. Scan any string for substrings appearing between two sets of characters — for example to parse out identifiers or metadata from a string of user-defined text.

Sweep can be dropped into a project as a general-purpose string scanning algorithm, or act as the base for custom, more high-level scanning implementations. It aims to complement the Swift standard library’s built-in string handling APIs, both in terms of its design, and also how its implemented in an efficient way in line with Swift’s various string conventions.


The easiest way to start using Sweep is to call the substrings method that it adds on top of StringProtocol — meaning that you can use it on both “normal” strings and Substring values.

Here’s an example in which we scan a string for HTML tags, and both identify the names of all tags that appear in the string, and also any text that should be rendered in bold:

import Sweep

let html = "<p>Hello, <b>this is bold</b>, right?</p>"
let tags = html.substrings(between: "<", and: ">")
print(tags) // ["p", "b", "/b", "/p"]

let boldText = html.substrings(between: "<b>", and: "</b>")
print(boldText) // ["this is bold"]

Sweep can also scan for different patterns, such as a prefix appearing at the start of the scanned string, or its end. Here we’re using those capabilities to identify headings in a string of Markdown-formatted text:

import Sweep

let markdown = """
## Section 1


## Section 2

let headings = markdown.substrings(between: [.prefix("## "), "\n## "],
                                   and: [.end, "\n"])

print(headings) // ["Section 1", "Section 2"]

Since Sweep was designed to fit right in alongside Swift’s built-in string APIs, it lets us compose more powerful string scanning algorithms using both built-in functionality and the APIs that Sweep adds — such as here where we’re parsing out an array of tags from a string written using a custom syntax:

import Sweep

let text = "{{tags: swift, programming, xcode}}"
let tagStrings = text.substrings(between: "{{tags: ", and: "}}")
let tags = tagStrings.flatMap { $0.components(separatedBy: ", ") }
print(tags) // ["swift", "programming", "xcode"]

Sweep was also designed to be highly efficient, and only makes a single pass through each string that it scans — regardless of how many different patterns you wish to scan for. In this example, we’re using two custom matchers to parse two pieces of metadata from a string:

import Sweep

let text = """
url: https://swiftbysundell.com
title: Swift by Sundell

var urls = [URL]()
var titles = [String]()

text.scan(using: [
    Matcher(identifiers: ["url: "], terminators: ["\n", .end]) { match, range in
        let string = String(match)
        let url = URL(string: string)
        url.flatMap { urls.append($0) }
    Matcher(identifiers: ["title: "], terminators: ["\n", .end]) { match, range in
        let string = String(match)

print(urls) // [https://swiftbysundell.com]
print(titles) // ["Swift by Sundell"]

Sweep is not only efficient in terms of complexity, it also has a very low memory overhead, thanks to it being built according to Swift’s modern string conventions — making full use of types like Substring and String.Index, and avoiding unnecessary copying and mutations when performing its scanning.


Sweep is distributed as a Swift package, and it’s recommended to install it using the Swift Package Manager, by declaring it as a dependency in your project’s Package.swift file:

.package(url: "https://github.com/JohnSundell/Sweep", from: "0.1.0")

For more information, please see the Swift Package Manager documentation.

Contributions & support

Sweep is developed completely in the open, and your contributions are more than welcome.

Before you start using Sweep in any of your projects, it’s highly recommended that you spend a few minutes familiarizing yourself with its documentation and internal implementation (it all fits in a single file!), so that you’ll be ready to tackle any issues or edge cases that you might encounter.

To learn more about the principles used to implement Sweep, check out “String parsing in Swift” on Swift by Sundell.

Sweep does not come with GitHub Issues-based support, and users are instead encouraged to become active participants in its continued development — by fixing any bugs that they encounter, or improving the documentation wherever it’s found to be lacking.

If you wish to make a change, open a Pull Request — even if it just contains a draft of the changes you’re planning, or a test that reproduces an issue — and we can discuss it further from there.

Hope you enjoy using Sweep! 😀


  • Swift Tools 5.0.0
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Last updated: Tue Aug 29 2023 17:23:38 GMT-0900 (Hawaii-Aleutian Daylight Time)