I am trying to figure out how to use NLTK's cascading chunker as per Chapter 7 of the NLTK book. Unfortunately, I'm running into a few issues when performing non-trivial chunking measures.

Let's start with this phrase:

"adventure movies between 2000 and 2015 featuring performances by daniel craig"

I am able to find all the relevant NPs when I use the following grammar:

grammar = "NP: {<DT>?<JJ>*<NN.*>+}"

However, I am not sure how to build nested structures with NLTK. The book gives the following format, but there are clearly a few things missing (e.g. How does one actually specify multiple rules?):

grammar = r"""
  NP: {<DT|JJ|NN.*>+}          # Chunk sequences of DT, JJ, NN
  PP: {<IN><NP>}               # Chunk prepositions followed by NP
  VP: {<VB.*><NP|PP|CLAUSE>+$} # Chunk verbs and their arguments
  CLAUSE: {<NP><VP>}           # Chunk NP, VP

In my case, I'd like to do something like the following:

grammar = "MEDIA: {<DT>?<JJ>*<NN.*>+}
           RELATION: {<V.*>}{<DT>?<JJ>*<NN.*>+}
           ENTITY: {<NN.*>}"

It occurs to me that a CFG might be a better fit for this, but I only became aware of NLTK's support for this function about 5 minutes ago (from this question), and it does not appear that much documentation for the feature exists.

So, assuming that I'd like to use a cascaded chunker for my task, what syntax would I need to use? Additionally, is it possible for me to specify specific words (e.g. "directed" or "acted") when using a chunker?


1 Answer 1


your grammar is correct!

grammar = """MEDIA: {<DT>?<JJ>*<NN.*>+}
           RELATION: {<V.*>}
           ENTITY: {<NN.*>}"""

by specifying


you are indicating that there are two ways to generate the RELATION chunk i.e. {<V.*>} or {<DT>?<JJ>*<NN.*>+}


grammar = """MEDIA: {<DT>?<JJ>*<NN.*>+}
               RELATION: {<V.*>}
               ENTITY: {<NN.*>}"""
    chunkParser = nltk.RegexpParser(grammar)
    tagged = nltk.pos_tag(nltk.word_tokenize("adventure movies between 2000 and 2015 featuring performances by daniel craig"))

    tree = chunkParser.parse(tagged)

    for subtree in tree.subtrees():
        if subtree.label() == "RELATION": 
            print("RELATION: "+str(subtree.leaves()))


RELATION: [('featuring', 'VBG')]

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.