WWDC24 SALE: Save 50% on all my Swift books and bundles! >>

How to lemmatize text using NLTagger

Swift version: 5.10

Paul Hudson    @twostraws   

Apple’s NaturalLanguage framework is able to lemmatize text for us, which is the process of converting words to the forms you would find in a dictionary – making plural nouns singular, finding the root forms of conjugated verbs, and so on, while also taking into account the context in which they are used.

To do this, first create an instance of NLTagger enabling its .lemma scheme, then call enumerateTags() on it to find all the root word forms. This will pass you the tag (the root word) if it exists, plus the range of the original text in the string.

So, you could lemmatize a whole sentence like this:

import NaturalLanguage

let text = "This is text with plurals such as geese, people, and millennia."
let tagger = NLTagger(tagSchemes: [.lemma])
tagger.string = text

tagger.enumerateTags(in: text.startIndex..<text.endIndex, unit: .word, scheme: .lemma) { tag, range in
    let stemForm = tag?.rawValue ?? String(text[range])
    print(stemForm, terminator: "")
    return true

Text lemmatized in this way will be lowercase, preserving any punctuation. So, that snippet will output “this be text with plural such as goose, person, and millennium.”

If you intend to lemmatize text frequently, consider making it an extension on String like this:

extension String {
    func lemmatized() -> String {
        let tagger = NLTagger(tagSchemes: [.lemma])
        tagger.string = self

        var result = [String]()

        tagger.enumerateTags(in: self.startIndex..<self.endIndex, unit: .word, scheme: .lemma) { tag, tokenRange in
            let stemForm = tag?.rawValue ?? String(self[tokenRange])
            return true

        return result.joined()

With that in place you can now lemmatize text easily:

let text = "This is text with plurals such as geese, people, and millennia."
Save 50% in my WWDC sale.

SAVE 50% To celebrate WWDC24, all our books and bundles are half price, so you can take your Swift knowledge further without spending big! Get the Swift Power Pack to build your iOS career faster, get the Swift Platform Pack to builds apps for macOS, watchOS, and beyond, or get the Swift Plus Pack to learn advanced design patterns, testing skills, and more.

Save 50% on all our books and bundles!

Available from iOS 12.0

Similar solutions…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

Buy Pro Swift Buy Pro SwiftUI Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let us know!

Average rating: 4.3/5

Unknown user

You are not logged in

Log in or create account

Link copied to your pasteboard.