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Python 3 Text Processing with NLTK 3 Cookbook
Python 3 Text Processing with NLTK 3 Cookbook

Python 3 Text Processing with NLTK 3 Cookbook: , Second Edition

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Python 3 Text Processing with NLTK 3 Cookbook

Chapter 2. Replacing and Correcting Words

In this chapter, we will cover the following recipes:

  • Stemming words
  • Lemmatizing words with WordNet
  • Replacing words matching regular expressions
  • Removing repeating characters
  • Spelling correction with Enchant
  • Replacing synonyms
  • Replacing negations with antonyms

Introduction

In this chapter, we will go over various word replacement and correction techniques. The recipes cover the gamut of linguistic compression, spelling correction, and text normalization. All of these methods can be very useful for preprocessing text before search indexing, document classification, and text analysis.

Stemming words

Stemming is a technique to remove affixes from a word, ending up with the stem. For example, the stem of cooking is cook, and a good stemming algorithm knows that the ing suffix can be removed. Stemming is most commonly used by search engines for indexing words. Instead of storing all forms of a word, a search engine can store only the stems, greatly reducing the size of index while increasing retrieval accuracy.

One of the most common stemming algorithms is the Porter stemming algorithm by Martin Porter. It is designed to remove and replace well-known suffixes of English words, and its usage in NLTK will be covered in the next section.

Note

The resulting stem is not always a valid word. For example, the stem of cookery is cookeri. This is a feature, not a bug.

How to do it...

NLTK comes with an implementation of the Porter stemming algorithm, which is very easy to use. Simply instantiate the PorterStemmer class and call the stem() method with the word you want to stem:

>&gt...

Lemmatizing words with WordNet

Lemmatization is very similar to stemming, but is more akin to synonym replacement. A lemma is a root word, as opposed to the root stem. So unlike stemming, you are always left with a valid word that means the same thing. However, the word you end up with can be completely different. A few examples will explain this.

Getting ready

Make sure that you have unzipped the wordnet corpus in nltk_data/corpora/wordnet. This will allow the WordNetLemmatizer class to access WordNet. You should also be familiar with the part-of-speech tags covered in the Looking up Synsets for a word in WordNet recipe of Chapter 1, Tokenizing Text and WordNet Basics.

How to do it...

We will use the WordNetLemmatizer class to find lemmas:

>>> from nltk.stem import WordNetLemmatizer
>>> lemmatizer = WordNetLemmatizer()
>>> lemmatizer.lemmatize('cooking')
'cooking'
>>> lemmatizer.lemmatize('cooking', pos='v')
&apos...

Replacing words matching regular expressions

Now, we are going to get into the process of replacing words. If stemming and lemmatization are a kind of linguistic compression, then word replacement can be thought of as error correction or text normalization.

In this recipe, we will replace words based on regular expressions, with a focus on expanding contractions. Remember when we were tokenizing words in Chapter 1, Tokenizing Text and WordNet Basics, and it was clear that most tokenizers had trouble with contractions? This recipe aims to fix this by replacing contractions with their expanded forms, for example, by replacing "can't" with "cannot" or "would've" with "would have".

Getting ready

Understanding how this recipe works will require a basic knowledge of regular expressions and the re module. The key things to know are matching patterns and the re.sub() function.

How to do it...

First, we need to define a number of replacement patterns...

Removing repeating characters

In everyday language, people are often not strictly grammatical. They will write things such as I looooooove it in order to emphasize the word love. However, computers don't know that "looooooove" is a variation of "love" unless they are told. This recipe presents a method to remove these annoying repeating characters in order to end up with a proper English word.

Getting ready

As in the previous recipe, we will be making use of the re module, and more specifically, backreferences. A backreference is a way to refer to a previously matched group in a regular expression. This will allow us to match and remove repeating characters.

How to do it...

We will create a class that has the same form as the RegexpReplacer class from the previous recipe. It will have a replace() method that takes a single word and returns a more correct version of that word, with the dubious repeating characters removed. This code can be found in replacers.py in the...

Introduction


In this chapter, we will go over various word replacement and correction techniques. The recipes cover the gamut of linguistic compression, spelling correction, and text normalization. All of these methods can be very useful for preprocessing text before search indexing, document classification, and text analysis.

Stemming words


Stemming is a technique to remove affixes from a word, ending up with the stem. For example, the stem of cooking is cook, and a good stemming algorithm knows that the ing suffix can be removed. Stemming is most commonly used by search engines for indexing words. Instead of storing all forms of a word, a search engine can store only the stems, greatly reducing the size of index while increasing retrieval accuracy.

One of the most common stemming algorithms is the Porter stemming algorithm by Martin Porter. It is designed to remove and replace well-known suffixes of English words, and its usage in NLTK will be covered in the next section.

Note

The resulting stem is not always a valid word. For example, the stem of cookery is cookeri. This is a feature, not a bug.

How to do it...

NLTK comes with an implementation of the Porter stemming algorithm, which is very easy to use. Simply instantiate the PorterStemmer class and call the stem() method with the word you want to stem:

>&gt...

Lemmatizing words with WordNet


Lemmatization is very similar to stemming, but is more akin to synonym replacement. A lemma is a root word, as opposed to the root stem. So unlike stemming, you are always left with a valid word that means the same thing. However, the word you end up with can be completely different. A few examples will explain this.

Getting ready

Make sure that you have unzipped the wordnet corpus in nltk_data/corpora/wordnet. This will allow the WordNetLemmatizer class to access WordNet. You should also be familiar with the part-of-speech tags covered in the Looking up Synsets for a word in WordNet recipe of Chapter 1, Tokenizing Text and WordNet Basics.

How to do it...

We will use the WordNetLemmatizer class to find lemmas:

>>> from nltk.stem import WordNetLemmatizer
>>> lemmatizer = WordNetLemmatizer()
>>> lemmatizer.lemmatize('cooking')
'cooking'
>>> lemmatizer.lemmatize('cooking', pos='v')
'cook'
>>> lemmatizer.lemmatize('cookbooks...
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Description

This book is intended for Python programmers interested in learning how to do natural language processing. Maybe you’ve learned the limits of regular expressions the hard way, or you’ve realized that human language cannot be deterministically parsed like a computer language. Perhaps you have more text than you know what to do with, and need automated ways to analyze and structure that text. This Cookbook will show you how to train and use statistical language models to process text in ways that are practically impossible with standard programming tools. A basic knowledge of Python and the basic text processing concepts is expected. Some experience with regular expressions will also be helpful.

What you will learn

  • Tokenize text into sentences, and sentences into words
  • Look up words in the WordNet dictionary
  • Apply spelling correction and word replacement
  • Access the builtin text corpora and create your own custom corpus
  • Tag words with parts of speech
  • Chunk phrases and recognize named entities
  • Grammatically transform phrases and chunks
  • Classify text and perform sentiment analysis
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Publication date : Aug 26, 2014
Length: 304 pages
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Language : English
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Table of Contents

11 Chapters
1. Tokenizing Text and WordNet Basics Chevron down icon Chevron up icon
2. Replacing and Correcting Words Chevron down icon Chevron up icon
3. Creating Custom Corpora Chevron down icon Chevron up icon
4. Part-of-speech Tagging Chevron down icon Chevron up icon
5. Extracting Chunks Chevron down icon Chevron up icon
6. Transforming Chunks and Trees Chevron down icon Chevron up icon
7. Text Classification Chevron down icon Chevron up icon
8. Distributed Processing and Handling Large Datasets Chevron down icon Chevron up icon
9. Parsing Specific Data Types Chevron down icon Chevron up icon
A. Penn Treebank Part-of-speech Tags Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

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It's a GREAT book!
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Good book
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Finally one of the best cookbook I've never read.Author is highly lucid and consistent in his explanation; He leaves nothing to chance and,when required, He delves into the topic.Material is extremely useful, code is very well designed! (I have read many texts on Python where code was catastrophic…) ; Excellent Support.
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It’s a very good information and with lots of hands on code. Really useful for ppl who are in there mid journey on NLP research
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