The ClassifierBasedPOSTagger class uses classification to do part-of-speech tagging. Features are extracted from words, and then passed to an internal classifier. The classifier classifies the features and returns a label, in this case, a part-of-speech tag. Classification will be covered in detail in Chapter 7, Text Classification.

The ClassifierBasedPOSTagger class is a subclass of ClassifierBasedTagger that implements a feature detector that combines many of the techniques of the previous taggers into a single feature set. The feature detector finds multiple length suffixes, does some regular expression matching, and looks at the unigram, bigram, and trigram history to produce a fairly complete set of features for each word. The feature sets it produces are used to train the internal classifier, and are used for classifying words into part-of-speech tags.