nltk.tokenize

NLTK Tokenizer Package

Tokenizers divide strings into lists of substrings. For example, tokenizers can be used to find the words and punctuation in a string:

>>> from nltk.tokenize import word_tokenize
>>> s = '''Good muffins cost $3.88\nin New York.  Please buy me
... two of them.\n\nThanks.'''
>>> word_tokenize(s)
['Good', 'muffins', 'cost', '$', '3.88', 'in', 'New', 'York', '.',
'Please', 'buy', 'me', 'two', 'of', 'them', '.', 'Thanks', '.']

This particular tokenizer requires the Punkt sentence tokenization models to be installed. NLTK also provides a simpler, regular-expression based tokenizer, which splits text on whitespace and punctuation:

>>> from nltk.tokenize import wordpunct_tokenize
>>> wordpunct_tokenize(s)
['Good', 'muffins', 'cost', '$', '3', '.', '88', 'in', 'New', 'York', '.',
'Please', 'buy', 'me', 'two', 'of', 'them', '.', 'Thanks', '.']

We can also operate at the level of sentences, using the sentence tokenizer directly as follows:

>>> from nltk.tokenize import sent_tokenize, word_tokenize
>>> sent_tokenize(s)
['Good muffins cost $3.88\nin New York.', 'Please buy me\ntwo of them.', 'Thanks.']
>>> [word_tokenize(t) for t in sent_tokenize(s)]
[['Good', 'muffins', 'cost', '$', '3.88', 'in', 'New', 'York', '.'],
['Please', 'buy', 'me', 'two', 'of', 'them', '.'], ['Thanks', '.']]

Caution: when tokenizing a Unicode string, make sure you are not using an encoded version of the string (it may be necessary to decode it first, e.g. with s.decode("utf8").

NLTK tokenizers can produce token-spans, represented as tuples of integers having the same semantics as string slices, to support efficient comparison of tokenizers. (These methods are implemented as generators.)

>>> from nltk.tokenize import WhitespaceTokenizer
>>> list(WhitespaceTokenizer().span_tokenize(s))
[(0, 4), (5, 12), (13, 17), (18, 23), (24, 26), (27, 30), (31, 36), (38, 44),
(45, 48), (49, 51), (52, 55), (56, 58), (59, 64), (66, 73)]

There are numerous ways to tokenize text. If you need more control over tokenization, see the other methods provided in this package.

For further information, please see Chapter 3 of the NLTK book.