How to parse a sentence using NSLinguisticTagger

Swift version: 5.1

Paul Hudson @twostraws May 28th 2019

If you're looking to parse natural language entered by a user, you're looking for NSLinguisticTagger: it automatically recognizes English words (and words in other languages too, if you ask) and tells you what kind of word it is. That is, this magic little class distinguishes between verbs, nouns, adjectives and so on, so you can focus on the important stuff: how do I (verb) this (noun)?

Here's an example to get you started:

let options = NSLinguisticTagger.Options.omitWhitespace.rawValue | NSLinguisticTagger.Options.joinNames.rawValue
let tagger = NSLinguisticTagger(tagSchemes: NSLinguisticTagger.availableTagSchemes(forLanguage: "en"), options: Int(options))

let inputString = "This is a very long test for you to try"
tagger.string = inputString

let range = NSRange(location: 0, length: inputString.utf16.count)
tagger.enumerateTags(in: range, scheme: .nameTypeOrLexicalClass, options: NSLinguisticTagger.Options(rawValue: options)) { tag, tokenRange, sentenceRange, stop in
    guard let range = Range(tokenRange, in: inputString) else { return }
    let token = inputString[range]
    print("\(tag): \(token)")
}

Available from iOS 5.0