/ Natural Language Processing

Text processing in Tensorflow

Text processing in Tensorflow

TensorFlow Text

TensorFlow Text provides a collection of text related classes and ops ready to use with TensorFlow 2.0. The library can perform the preprocessing regularly required by text-based models, and includes other features useful for sequence modeling not provided by core TensorFlow.

The benefit of using these ops in your text preprocessing is that they are done in the TensorFlow graph. You do not need to worry about tokenization in training being different than the tokenization at inference, or managing preprocessing scripts.

Eager Execution

TensorFlow Text is compatible with both TensorFlow eager mode and graph mode.

import tensorflow as tf
import tensorflow_text as text
tf.enable_eager_execution()

Unicode

Most ops expect that the strings are in UTF-8. If your using a different
encoding, you can use the core tensorflow transcode op to transcode into UTF-8.
You can also use the same op to coerce your string to structurally valid UTF-8
if your input could be invalid.

docs = tf.constant([u'Everything not saved will be lost.'.encode('UTF-16-BE'),
                    u'Sad☹'.encode('UTF-16-BE')])
utf8_docs = tf.strings.unicode_transcode(docs, input_encoding='UTF-16-BE',
                                         output_encoding='UTF-8')

Normalization

When dealing with different sources of text, it's important that the same words
are recognized to be identical. A common technique for case-insensitive matching
in Unicode is case folding (similar to lower-casing). (Note that case folding
internally applies NFKC normalization.)

We also provide Unicode normalization ops for transforming strings into a
canonical representation of characters, with Normalization Form KC being the
default (NFKC).

print(text.case_fold_utf8(['Everything not saved will be lost.']))
print(text.normalize_utf8(['Äffin']))
print(text.normalize_utf8(['Äffin'], 'nfkd'))
tf.Tensor(['everything not saved will be lost.'], shape=(1,), dtype=string)
tf.Tensor(['\xc3\x84ffin'], shape=(1,), dtype=string)
tf.Tensor(['A\xcc\x88ffin'], shape=(1,), dtype=string)

Tokenization

Tokenization is the process of breaking up a string into tokens. Commonly, these
tokens are words, numbers, and/or punctuation.

The main interfaces are Tokenizer and TokenizerWithOffsets which each have a
single method tokenize and tokenizeWithOffsets respectively. There are
multiple implementing tokenizers available now. Each of these implement
TokenizerWithOffsets (which extends Tokenizer) which includes an option for
getting byte offsets into the original string. This allows the caller to know
the bytes in the original string the token was created from.

All of the tokenizers return RaggedTensors with the inner-most dimension of
tokens mapping to the original individual strings. As a result, the resulting
shape's rank is increased by one. Please review the ragged tensor guide if you
are unfamiliar with them. https://www.tensorflow.org/guide/ragged_tensors

WhitespaceTokenizer

This is a basic tokenizer that splits UTF-8 strings on ICU defined whitespace
characters (eg. space, tab, new line).

tokenizer = text.WhitespaceTokenizer()
tokens = tokenizer.tokenize(['everything not saved will be lost.', u'Sad☹'.encode('UTF-8')])
print(tokens.to_list())
[['everything', 'not', 'saved', 'will', 'be', 'lost.'], ['Sad\xe2\x98\xb9']]

UnicodeScriptTokenizer

This tokenizer splits UTF-8 strings based on Unicode script boundaries. The
script codes used correspond to International Components for Unicode (ICU)
UScriptCode values. See: http://icu-project.org/apiref/icu4c/uscript_8h.html

In practice, this is similar to the WhitespaceTokenizer with the most apparent
difference being that it will split punctuation (USCRIPT_COMMON) from language
texts (eg. USCRIPT_LATIN, USCRIPT_CYRILLIC, etc) while also separating language
texts from each other.

tokenizer = text.UnicodeScriptTokenizer()
tokens = tokenizer.tokenize(['everything not saved will be lost.',
                             u'Sad☹'.encode('UTF-8')])
print(tokens.to_list())
[['everything', 'not', 'saved', 'will', 'be', 'lost', '.'],
 ['Sad', '\xe2\x98\xb9']]

Unicode split

When tokenizing languages without whitespace to segment words, it is common to
just split by character, which can be accomplished using the
unicode_split
op found in core.

tokens = tf.strings.unicode_split([u"仅今年前".encode('UTF-8')], 'UTF-8')
print(tokens.to_list())
[['\xe4\xbb\x85', '\xe4\xbb\x8a', '\xe5\xb9\xb4', '\xe5\x89\x8d']]

Offsets

When tokenizing strings, it is often desired to know where in the original
string the token originated from. For this reason, each tokenizer which
implements TokenizerWithOffsets has a tokenize_with_offsets method that will
return the byte offsets along with the tokens. The offset_starts lists the bytes
in the original string each token starts at, and the offset_limits lists the
bytes where each token ends at.

tokenizer = text.UnicodeScriptTokenizer()
(tokens, offset_starts, offset_limits) = tokenizer.tokenize_with_offsets(
    ['everything not saved will be lost.', u'Sad☹'.encode('UTF-8')])
print(tokens.to_list())
print(offset_starts.to_list())
print(offset_limits.to_list())
[['everything', 'not', 'saved', 'will', 'be', 'lost', '.'],
 ['Sad', '\xe2\x98\xb9']]
[[0, 11, 15, 21, 26, 29, 33], [0, 3]]
[[10, 14, 20, 25, 28, 33, 34], [3, 6]]

TF.Data Example

Tokenizers work as expected with the tf.data API. A simple example is provided
below.

docs = tf.data.Dataset.from_tensor_slices([['Never tell me the odds.'],
                                           ["It's a trap!"]])
tokenizer = text.WhitespaceTokenizer()
tokenized_docs = docs.map(lambda x: tokenizer.tokenize(x))
iterator = tokenized_docs.make_one_shot_iterator()
print(iterator.get_next().to_list())
print(iterator.get_next().to_list())
[['Never', 'tell', 'me', 'the', 'odds.']]
[["It's", 'a', 'trap!']]

Other Text Ops

TF.Text packages other useful preprocessing ops. We will review a couple below.

Wordshape

A common feature used in some natural language understanding models is to see
if the text string has a certain property. For example, a sentence breaking
model might contain features which check for word capitalization or if a
punctuation character is at the end of a string.

Wordshape defines a variety of useful regular expression based helper functions
for matching various relevant patterns in your input text. Here are a few
examples.

tokenizer = text.WhitespaceTokenizer()
tokens = tokenizer.tokenize(['Everything not saved will be lost.',
                             u'Sad☹'.encode('UTF-8')])

# Is capitalized?
f1 = text.wordshape(tokens, text.WordShape.HAS_TITLE_CASE)
# Are all letters uppercased?
f2 = text.wordshape(tokens, text.WordShape.IS_UPPERCASE)
# Does the token contain punctuation?
f3 = text.wordshape(tokens, text.WordShape.HAS_SOME_PUNCT_OR_SYMBOL)
# Is the token a number?
f4 = text.wordshape(tokens, text.WordShape.IS_NUMERIC_VALUE)

print(f1.to_list())
print(f2.to_list())
print(f3.to_list())
print(f4.to_list())
[[True, False, False, False, False, False], [True]]
[[False, False, False, False, False, False], [False]]
[[False, False, False, False, False, True], [True]]
[[False, False, False, False, False, False], [False]]

N-grams & Sliding Window

N-grams are sequential words given a sliding window size of n. When combining
the tokens, there are three reduction mechanisms supported. For text, you would
want to use Reduction.STRING_JOIN which appends the strings to each other.
The default separator character is a space, but this can be changed with the
string_separater argument.

The other two reduction methods are most often used with numerical values, and
these are Reduction.SUM and Reduction.MEAN.

tokenizer = text.WhitespaceTokenizer()
tokens = tokenizer.tokenize(['Everything not saved will be lost.',
                             u'Sad☹'.encode('UTF-8')])

# Ngrams, in this case bi-gram (n = 2)
bigrams = text.ngrams(tokens, 2, reduction_type=text.Reduction.STRING_JOIN)

print(bigrams.to_list())
[['Everything not', 'not saved', 'saved will', 'will be', 'be lost.'], []]

Installation

Install using PIP

pip install -U tensorflow-text

GitHub