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Many methods on String return Substring. I found myself wanting to use a Regex to parse part of one of these substrings recently, but found that Regex only works on a full String.

Is there a reason that Regex could not work with Substrings as well?

8 Likes
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@bjhomer What APIs are you exactly missing? I've just tested and the ones I personally would expect seem to work with Substring, too. Running this code in a playground worked as expected:

import Foundation

let string: String = "Hello, world!"
let substring: Substring = string.split(separator: ", ").first!

let regex = /abc/
string.matches(of: regex) // => []
substring.matches(of: regex) // => []

Can you be more specific? To me it seems like Substring is already supported ...

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Ah, interesting. What I'm actually seeing is that although regexes support both String and Substring arguments, they don't support StringProtocol. I have a bunch of functions like this:

func parseLine(_ line: some StringProtocol) {
  let regex = /You earned (\d+) points by collecting (\d+) gems/
  if let match = try regex.wholeMatch(in: line) {
    // use the match here
  }
}

This makes it easy for me to pass it a Substring that arises as part of parsing a file line-by-line, but I can also test it individually by passing an individual string. But I get the error:

// error: No exact matches in call to instance method 'wholeMatch'

… because wholeMatch doesn't take a some StringProtocol. It only has concrete overrides for String and Substring.

I wonder if it would be possible to define the regex operations in terms of StringProtocol

7 Likes
4

This idea was considered and dismissed by one of the authors of SE-0357 in its review thread:

Instead of using StringProtocol, why not just use Substring? You can easily create a Substring from a String like so:

let substring = "string"[...]