Written by Mattt Thompson on

NSLinguisticTagger is a veritable Swiss Army Knife of linguistic functionality, with the ability to tokenize natural language strings into words, determine their part-of-speech & stem, extract names of people, places, & organizations, and tell you the languages & respective writing system used in the string.

For most of us, this is far more power than we know what to do with. But perhaps this is just for lack sufficient opportunity to try. After all, almost every application deals with natural language in one way or another--perhaps NSLinguisticTagger could add a new level of polish, or enable brand new features entirely.

Introduced with iOS 5, NSLinguisticTagger is a contemporary to Siri, raising speculation that it was a byproduct of the personal assistant's development.

Consider a typical question we might ask Siri:

What is the weather in San Francisco?

Computers are a long ways off from "understanding" this question literally, but with a few simple tricks, we can do a reasonable job understanding the intention of the question:

NSString *question = @"What is the weather in San Francisco?";
NSLinguisticTaggerOptions options = NSLinguisticTaggerOmitWhitespace | NSLinguisticTaggerOmitPunctuation | NSLinguisticTaggerJoinNames;
NSLinguisticTagger *tagger = [[NSLinguisticTagger alloc] initWithTagSchemes: [NSLinguisticTagger availableTagSchemesForLanguage:@"en"] options:options];
tagger.string = question;
[tagger enumerateTagsInRange:NSMakeRange(0, [question length]) scheme:NSLinguisticTagSchemeNameTypeOrLexicalClass options:options usingBlock:^(NSString *tag, NSRange tokenRange, NSRange sentenceRange, BOOL *stop) {
    NSString *token = [question substringWithRange:tokenRange];
    NSLog(@"%@: %@", token, tag);

This code would print the following:

What: Pronoun is: Verb the: Determiner weather: Noun in: Preposition San Francisco: PlaceName

If we filter on nouns, verbs, and place name, we get [is, weather, San Francisco].

Just based on this alone, or perhaps in conjunction with something like the Latent Semantic Mapping framework, we can conclude that a reasonable course of action would be to make an API request to determine the current weather conditions in San Francisco.

Tagging Schemes

NSLinguisticTagger can be configured to tag different kinds of information by specifying any of the following tagging schemes:

  • NSLinguisticTagSchemeTokenType: Classifies tokens according to their broad type: word, punctuation, whitespace, etc.
  • NSLinguisticTagSchemeLexicalClass: Classifies tokens according to class: part of speech for words, type of punctuation or whitespace, etc.
  • NSLinguisticTagSchemeNameType: Classifies tokens as to whether they are part of named entities of various types or not.
  • NSLinguisticTagSchemeNameTypeOrLexicalClass: Follows NSLinguisticTagSchemeNameType for names, and NSLinguisticTagSchemeLexicalClass for all other tokens.

Here's a list of the various token types associated with each scheme (NSLinguisticTagSchemeNameTypeOrLexicalClass, as the name implies, is the union between NSLinguisticTagSchemeNameType & NSLinguisticTagSchemeLexicalClass):

NSLinguisticTagSchemeTokenType NSLinguisticTagSchemeLexicalClass NSLinguisticTagSchemeNameType
  • NSLinguisticTagWord
  • NSLinguisticTagPunctuation
  • NSLinguisticTagWhitespace
  • NSLinguisticTagOther
  • NSLinguisticTagNoun
  • NSLinguisticTagVerb
  • NSLinguisticTagAdjective
  • NSLinguisticTagAdverb
  • NSLinguisticTagPronoun
  • NSLinguisticTagDeterminer
  • NSLinguisticTagParticle
  • NSLinguisticTagPreposition
  • NSLinguisticTagNumber
  • NSLinguisticTagConjunction
  • NSLinguisticTagInterjection
  • NSLinguisticTagClassifier
  • NSLinguisticTagIdiom
  • NSLinguisticTagOtherWord
  • NSLinguisticTagSentenceTerminator
  • NSLinguisticTagOpenQuote
  • NSLinguisticTagCloseQuote
  • NSLinguisticTagOpenParenthesis
  • NSLinguisticTagCloseParenthesis
  • NSLinguisticTagWordJoiner
  • NSLinguisticTagDash
  • NSLinguisticTagOtherPunctuation
  • NSLinguisticTagParagraphBreak
  • NSLinguisticTagOtherWhitespace
  • NSLinguisticTagPersonalName
  • NSLinguisticTagPlaceName
  • NSLinguisticTagOrganizationName

So for basic tokenization, use NSLinguisticTagSchemeTokenType, which will allow you to distinguish between words and whitespace or punctuation. For information like part-of-speech, or differentiation between different parts of speech, NSLinguisticTagSchemeLexicalClass is your new bicycle.

Continuing with the tagging schemes:

  • NSLinguisticTagSchemeLemma: This tag scheme supplies a stem forms of the words, if known.
  • NSLinguisticTagSchemeLanguage: Tags tokens according to their script. The tag values will be standard language abbreviations such as "en", "fr", "de", etc., as used with the NSOrthography class. Note that the tagger generally attempts to determine the language of text at the level of an entire sentence or paragraph, rather than word by word.
  • NSLinguisticTagSchemeScript: Tags tokens according to their script. The tag values will be standard script abbreviations such as "Latn", "Cyrl", "Jpan", "Hans", "Hant", etc.

As demonstrated in the example above, first you initialize an NSLinguisticTagger with an array of all of the different schemes that you wish to use, and then assign or enumerate each of the tags after specifying the tagger's input string.

Tagging Options

In addition to the available tagging schemes, there are several options you can pass to NSLinguisticTagger (combined with bitwise OR |) to slightly change its behavior:

  • NSLinguisticTaggerOmitWords
  • NSLinguisticTaggerOmitPunctuation
  • NSLinguisticTaggerOmitWhitespace
  • NSLinguisticTaggerOmitOther

Each of these options omit the broad categories of tags described. For example, NSLinguisticTagSchemeLexicalClass, which distinguishes between many different kinds of punctuation, all of those would be omitted with NSLinguisticTaggerOmitPunctuation. This is preferable to manually filtering these tag types in enumeration blocks or with predicates.

The last option is specific to NSLinguisticTagSchemeNameType:

  • NSLinguisticTaggerJoinNames

By default, each token in a name is treated as separate instances. In many circumstances, it makes sense to treat names like "San Francisco" as a single token, rather than two separate tokens. Passing this token makes this so.

Natural language is woefully under-utilized in user interface design on mobile devices. When implemented effectively, a single utterance from the user can achieve the equivalent of a handful of touch interactions, in a fraction of the time.

Sure, it's not easy, but if we spent a fraction of the time we use to make our visual interfaces pixel-perfect, we could completely re-imagine how users best interact with apps and devices. And with NSLinguisticTagger, it's never been easier to get started.