Why is Regex slower than using a bunch of String.contains?

[tera]

@wadetregaskis

Oh, I see, if you merely import Foundation then it changes the implementation of strings.

Yep

@wadetregaskis

Unfortunately, the "pure" Swift version is an order of magnitude slower than the Foundation version

I don't see this, not in your test at least... the async/await version takes 23 seconds regardless of whether I'm using String.contains or String.contains2:~

Wrong code hidden, see below

// in a file without `import Foundation`

extension String {
    func contains2(_ v: String) -> Bool {
        contains(v)
    }
    func contains2(_ v: Regex<Substring>) -> Bool {
        contains(v)
    }
}

@wadetregaskis

Plus, how realistic is to not have Foundation imported anyway? I imagine there's few non-trivial Swift programs which don't use Foundation.

It is possible even for macOS/iOS apps – by bringing small key fragments to swift files that don't have import Foundation – but not many people would bother doing that.

Oops. That's factually wrong. I assumed that a separate file with no import Foundation in it would choose non-foundation implementation for, say, String.contains. That's not the case if import Foundation being used in other files of the same project.

Correct answer then – no, it's not realistic for iOS apps and non server macOS apps to not use Foundation, the situation for server apps is different.