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# Natural Language Toolkit: Chunkers # # Copyright (C) 2001-2012 NLTK Project # Author: Steven Bird <sb@csse.unimelb.edu.au> # Edward Loper <edloper@gradient.cis.upenn.edu> # URL: <http://www.nltk.org/> # For license information, see LICENSE.TXT #
Classes and interfaces for identifying non-overlapping linguistic groups (such as base noun phrases) in unrestricted text. This task is called "chunk parsing" or "chunking", and the identified groups are called "chunks". The chunked text is represented using a shallow tree called a "chunk structure." A chunk structure is a tree containing tokens and chunks, where each chunk is a subtree containing only tokens. For example, the chunk structure for base noun phrase chunks in the sentence "I saw the big dog on the hill" is::
(SENTENCE: (NP: <I>) <saw> (NP: <the> <big> <dog>) <on> (NP: <the> <hill>))
To convert a chunk structure back to a list of tokens, simply use the chunk structure's ``leaves()`` method.
This module defines ``ChunkParserI``, a standard interface for chunking texts; and ``RegexpChunkParser``, a regular-expression based implementation of that interface. It also defines ``ChunkScore``, a utility class for scoring chunk parsers.
RegexpChunkParser =================
``RegexpChunkParser`` is an implementation of the chunk parser interface that uses regular-expressions over tags to chunk a text. Its ``parse()`` method first constructs a ``ChunkString``, which encodes a particular chunking of the input text. Initially, nothing is chunked. ``parse.RegexpChunkParser`` then applies a sequence of ``RegexpChunkRule`` rules to the ``ChunkString``, each of which modifies the chunking that it encodes. Finally, the ``ChunkString`` is transformed back into a chunk structure, which is returned.
``RegexpChunkParser`` can only be used to chunk a single kind of phrase. For example, you can use an ``RegexpChunkParser`` to chunk the noun phrases in a text, or the verb phrases in a text; but you can not use it to simultaneously chunk both noun phrases and verb phrases in the same text. (This is a limitation of ``RegexpChunkParser``, not of chunk parsers in general.)
RegexpChunkRules ----------------
A ``RegexpChunkRule`` is a transformational rule that updates the chunking of a text by modifying its ``ChunkString``. Each ``RegexpChunkRule`` defines the ``apply()`` method, which modifies the chunking encoded by a ``ChunkString``. The ``RegexpChunkRule`` class itself can be used to implement any transformational rule based on regular expressions. There are also a number of subclasses, which can be used to implement simpler types of rules:
- ``ChunkRule`` chunks anything that matches a given regular expression. - ``ChinkRule`` chinks anything that matches a given regular expression. - ``UnChunkRule`` will un-chunk any chunk that matches a given regular expression. - ``MergeRule`` can be used to merge two contiguous chunks. - ``SplitRule`` can be used to split a single chunk into two smaller chunks. - ``ExpandLeftRule`` will expand a chunk to incorporate new unchunked material on the left. - ``ExpandRightRule`` will expand a chunk to incorporate new unchunked material on the right.
Tag Patterns ~~~~~~~~~~~~
A ``RegexpChunkRule`` uses a modified version of regular expression patterns, called "tag patterns". Tag patterns are used to match sequences of tags. Examples of tag patterns are::
r'(<DT>|<JJ>|<NN>)+' r'<NN>+' r'<NN.*>'
The differences between regular expression patterns and tag patterns are:
- In tag patterns, ``'<'`` and ``'>'`` act as parentheses; so ``'<NN>+'`` matches one or more repetitions of ``'<NN>'``, not ``'<NN'`` followed by one or more repetitions of ``'>'``. - Whitespace in tag patterns is ignored. So ``'<DT> | <NN>'`` is equivalant to ``'<DT>|<NN>'`` - In tag patterns, ``'.'`` is equivalant to ``'[^{}<>]'``; so ``'<NN.*>'`` matches any single tag starting with ``'NN'``.
The function ``tag_pattern2re_pattern`` can be used to transform a tag pattern to an equivalent regular expression pattern.
Efficiency ----------
Preliminary tests indicate that ``RegexpChunkParser`` can chunk at a rate of about 300 tokens/second, with a moderately complex rule set.
There may be problems if ``RegexpChunkParser`` is used with more than 5,000 tokens at a time. In particular, evaluation of some regular expressions may cause the Python regular expression engine to exceed its maximum recursion depth. We have attempted to minimize these problems, but it is impossible to avoid them completely. We therefore recommend that you apply the chunk parser to a single sentence at a time.
Emacs Tip ---------
If you evaluate the following elisp expression in emacs, it will colorize a ``ChunkString`` when you use an interactive python shell with emacs or xemacs ("C-c !")::
(let () (defconst comint-mode-font-lock-keywords '(("<[^>]+>" 0 'font-lock-reference-face) ("[{}]" 0 'font-lock-function-name-face))) (add-hook 'comint-mode-hook (lambda () (turn-on-font-lock))))
You can evaluate this code by copying it to a temporary buffer, placing the cursor after the last close parenthesis, and typing "``C-x C-e``". You should evaluate it before running the interactive session. The change will last until you close emacs.
Unresolved Issues -----------------
If we use the ``re`` module for regular expressions, Python's regular expression engine generates "maximum recursion depth exceeded" errors when processing very large texts, even for regular expressions that should not require any recursion. We therefore use the ``pre`` module instead. But note that ``pre`` does not include Unicode support, so this module will not work with unicode strings. Note also that ``pre`` regular expressions are not quite as advanced as ``re`` ones (e.g., no leftward zero-length assertions).
:type CHUNK_TAG_PATTERN: regexp :var CHUNK_TAG_PATTERN: A regular expression to test whether a tag pattern is valid. """
tree2conlltags, tree2conllstr, tree2conlltags, ieerstr2tree)
# Standard treebank POS tagger
""" Use NLTK's currently recommended named entity chunker to chunk the given list of tagged tokens. """ if binary: chunker_pickle = _BINARY_NE_CHUNKER else: chunker_pickle = _MULTICLASS_NE_CHUNKER chunker = load(chunker_pickle) return chunker.parse(tagged_tokens)
""" Use NLTK's currently recommended named entity chunker to chunk the given list of tagged sentences, each consisting of a list of tagged tokens. """ if binary: chunker_pickle = _BINARY_NE_CHUNKER else: chunker_pickle = _MULTICLASS_NE_CHUNKER chunker = load(chunker_pickle) return chunker.batch_parse(tagged_sentences)
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