Manipulating text at scale¶

This section introduces you to regular expressions for manipulating text and how to apply the same procedure to several files.

Ideally, Python should enable you to manipulate text at scale, that is, to apply the same procedure to ten, hundred or thousand text files with the same effort.

To do so, we must be able to define more flexible patterns than the fixed strings that we used previously with the replace() method, while opening and closing files automatically.

This capability is provided by Python modules for regular expressions and file handling.

After reading this section, you should know:

how to manipulate multiple text files using Python
how to define simple patterns using regular expressions
how to save the results

Regular expressions¶

Regular expressions are a “language” that allows defining search patterns.

These patterns can be used to find or to find and replace patterns in Python string objects.

As opposed to fixed strings, regular expressions allow defining wildcard characters that stand in for any character, quantifiers that match sequences of repeated characters, and much more.

Python allows using regular expressions through its re module.

We can activate this module using the import command.

import re

Let’s begin by loading the text file, reading its contents, assigning the last 2000 characters to the variable extract and printing out the result.

# Define a path to the file, open the file for (r)eading using utf-8 encoding
with open(file='data/WP_1990-08-10-25A.txt', mode='r', encoding='utf-8') as file:

    # Read the file contents using the .read() method
    text = file.read()

# Get the *last* 2000 characters – note the minus sign before the number
extract = text[-2000:]

# Print the result
print(extract)

 guardian of democracy .., also works as a deterrent to ter;;, rorists.
   If the United States and all its; might wasn't viewed as looking- out for smaller countries’ inter- ; ests, Rogers said, "things could get completely out of hand.;- Anybody could do anything/; Terrorism would spread like a cancer.”	,
   Poor people may be the first'-'* to feel repercussions from Iraq's invasion of Kuwait. Al->." ready, the price of a tankful of gasoline has increased a dollar— or two at many pumps in the region.
   "This is going to hurt the. poor man and the U.S. car tor.... dustry,” Rogers predicted.
   “Even if they ration gas, ;lt won’t matter to rich people*”...;, said Belk, from the VA nursing-,:.’ home. “They will buy it on the black market. But if you don't,7 have much money to begin with;-’; it’s going to make a difference/*—.
   Gerald Dunn, a federal gov*” - eminent employee from Alex-* 1 andria, said that he believed'"" that most middle- and upper-in-"“ come families wouldn’t even no-.; — tice the increase in gas prices.-'; Besides, he said, it was a small -price for them to pay while the nation defended a "country un-' der attack.”	’’
   Saddam Hussein, alternatively described as a madman, bully-”" and dictator, drew passionate'"''. repudiation.
   "No way can we let him get' away with this,” said Robert Stout, a vice president at Smithy Braedon, referring to . the Iraqi president’s decision to.... annex Kuwait.	,
   “Saddam Hussein really,,,,.* scares me," said Falls Church,,,, music instructor Joseph Moq-, ton. "I fear we have a madmaq,,,„; out there and he is not going to;;,* stop at Kuwait. These days, it takes is a nuclear weapon; small enough to fit in a suit-.;.; case.”	—
   ”1 just hope they stop him — quick,” said Viola Anderson, a"-"
retired hotel employee in tlie	
District. "I really feel things could turn terrible, if we donfr stop him soon.”	>•'«■«
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

As you can see, the text has a lot of errors from optical character recognition, mainly in the form of sequences such as .... and ,,,,.

Let’s compile our first regular expression that searches for sequences of two or more full stops.

This is done using the compile() function from the re module.

The compile() function takes a string as an input.

Note that we attach the prefix r to the string. This tells Python to store the string in ‘raw’ format. This means that the string is stored as it appears.

# Compile a regular expression and assign it to the variable 'stops'
stops = re.compile(r'\.{2,}')

# Let's check the type of the regular expression!
type(stops)

re.Pattern

Let’s unpack this regular expression a bit.

The regular expression is defined using a Python string, as indicated by the single quotation marks ' '.
We need a backslash \ in front of our full stop .. The backslash tells Python that we are really referring to a full stop, because regular expressions use a full stop as a wildcard character that can stand in for any character.
The curly brackets { } instruct the regular expression to search for instances of the previous item \. (our actual full stop) that occur two or more times (2,). This (hopefully) preserves true uses of a full stop!

In plain language, we tell the regular expression to search for occurrences of two or more full stops.

To apply this regular expression to some text, we will use the sub() method of our newly-defined regular expression object stops.

The sub() method takes two arguments:

repl: A string containing a string that is used to replace possible matches.
string: A string object to be searched for matches.

The method returns the modified string object.

Let’s apply our regular expression to the string stored under the variable extract.

# Apply the regular expression to the text under 'extract' and save the output
# to the same variable, essentially overwriting the old text.
extract = stops.sub(repl='', string=extract)

# Print the text to examine the result
print(extract)

 guardian of democracy , also works as a deterrent to ter;;, rorists.
   If the United States and all its; might wasn't viewed as looking- out for smaller countries’ inter- ; ests, Rogers said, "things could get completely out of hand.;- Anybody could do anything/; Terrorism would spread like a cancer.”	,
   Poor people may be the first'-'* to feel repercussions from Iraq's invasion of Kuwait. Al->." ready, the price of a tankful of gasoline has increased a dollar— or two at many pumps in the region.
   "This is going to hurt the. poor man and the U.S. car tor dustry,” Rogers predicted.
   “Even if they ration gas, ;lt won’t matter to rich people*”;, said Belk, from the VA nursing-,:.’ home. “They will buy it on the black market. But if you don't,7 have much money to begin with;-’; it’s going to make a difference/*—.
   Gerald Dunn, a federal gov*” - eminent employee from Alex-* 1 andria, said that he believed'"" that most middle- and upper-in-"“ come families wouldn’t even no-.; — tice the increase in gas prices.-'; Besides, he said, it was a small -price for them to pay while the nation defended a "country un-' der attack.”	’’
   Saddam Hussein, alternatively described as a madman, bully-”" and dictator, drew passionate'"''. repudiation.
   "No way can we let him get' away with this,” said Robert Stout, a vice president at Smithy Braedon, referring to . the Iraqi president’s decision to annex Kuwait.	,
   “Saddam Hussein really,,,,.* scares me," said Falls Church,,,, music instructor Joseph Moq-, ton. "I fear we have a madmaq,,,„; out there and he is not going to;;,* stop at Kuwait. These days, it takes is a nuclear weapon; small enough to fit in a suit-.;.; case.”	—
   ”1 just hope they stop him — quick,” said Viola Anderson, a"-"
retired hotel employee in tlie	
District. "I really feel things could turn terrible, if we donfr stop him soon.”	>•'«■«
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

As you can see, the sequences of full stops are gone.

We can make our regular expression even more powerful by adding alternatives.

Let’s compile another regular expression and store it under the variable punct.

# Compile a regular expression and assign it to the variable 'punct'
punct = re.compile(r'(\.|,){2,}')

What’s new here are the parentheses ( ) and the vertical bar | between them, which separates our actual full stop \. and the comma ,.

The characters surrounded by parentheses and separated by a vertical bar mark alternatives.

In plain English, we tell the regular expression to search for occurrences of two or more full stops or commas.

Let’s apply our new pattern to the text under extract.

To ensure the pattern works as intended, let’s retrieve the original text from the text variable and assign it to the variable extract to overwrite our previous edits.

# "Reset" the extract variable by taking the last 2000 characters of the original string
extract = text[-2000:]

# Apply the regular expression
extract = punct.sub(repl='', string=extract)

# Print out the result
print(extract)

 guardian of democracy  also works as a deterrent to ter;;, rorists.
   If the United States and all its; might wasn't viewed as looking- out for smaller countries’ inter- ; ests, Rogers said, "things could get completely out of hand.;- Anybody could do anything/; Terrorism would spread like a cancer.”	,
   Poor people may be the first'-'* to feel repercussions from Iraq's invasion of Kuwait. Al->." ready, the price of a tankful of gasoline has increased a dollar— or two at many pumps in the region.
   "This is going to hurt the. poor man and the U.S. car tor dustry,” Rogers predicted.
   “Even if they ration gas, ;lt won’t matter to rich people*”;, said Belk, from the VA nursing-,:.’ home. “They will buy it on the black market. But if you don't,7 have much money to begin with;-’; it’s going to make a difference/*—.
   Gerald Dunn, a federal gov*” - eminent employee from Alex-* 1 andria, said that he believed'"" that most middle- and upper-in-"“ come families wouldn’t even no-.; — tice the increase in gas prices.-'; Besides, he said, it was a small -price for them to pay while the nation defended a "country un-' der attack.”	’’
   Saddam Hussein, alternatively described as a madman, bully-”" and dictator, drew passionate'"''. repudiation.
   "No way can we let him get' away with this,” said Robert Stout, a vice president at Smithy Braedon, referring to . the Iraqi president’s decision to annex Kuwait.	,
   “Saddam Hussein really* scares me," said Falls Church music instructor Joseph Moq-, ton. "I fear we have a madmaq„; out there and he is not going to;;,* stop at Kuwait. These days, it takes is a nuclear weapon; small enough to fit in a suit-.;.; case.”	—
   ”1 just hope they stop him — quick,” said Viola Anderson, a"-"
retired hotel employee in tlie	
District. "I really feel things could turn terrible, if we donfr stop him soon.”	>•'«■«
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

Success! The sequences of full stops and commas can be removed using a single regular expression.

The more irregular sequences resulting from optical character recognition errors in extract, such as '-'*, ->.", /*—., -"“ and '"''. are much harder to capture.

Capturing these patterns would require defining more complex regular expressions, which are harder to write. Their complexity is, however, what makes regular expressions so powerful, but at the same time, learning how to use them takes time and patience.

It is therefore a good idea to use a service such as regex101.com to learn the basics of regular expressions.

In practice, coming up with regular expressions that cover as many matches as possible is particularly hard.

Capturing most of the errors – and perhaps distributing the manipulations over a series of steps in a pipeline – can already help prepare the text for further processing or analysis.

However, keep in mind that in order to identify patterns for manipulating text programmatically, you should always look at more than one text in your corpus.

Processing multiple files¶

Many corpora contain texts in multiple files.

To make manipulating high volumes of text as efficient as possible, we must open the files, read their contents, perform the requested operations and close them programmatically.

This procedure is made fairly simple using the Path class from Python’s pathlib module.

Let’s import the class first. Using the command from with import allows us to import only a part of the pathlib module, namely the Path class. This is useful if you only need some feature contained in a Python module or library.

from pathlib import Path

The Path class encodes information about paths in a directory structure.

What’s particularly great about the Path class is that it can automatically infer what kinds of paths your operating system uses.

Here the problem is that operating systems such as Windows, Linux and Mac OS X have different file system paths.

Using the Path class allows us to avoid a lot of trouble, particularly if we want our code to run on different operating systems.

Our repository contains a directory named data, which contains the text files that we have been working with recently.

Let’s initialise a Path object that points towards this directory by providing a string with the directory name to the Path class. We assign the object to the variable corpus_dir.

# Create a Path object that points towards the directory 'data' and assign
# the object to the variable 'corpus_dir'
corpus_dir = Path('data')

The Path object stored under corpus_dir has various useful methods and attributes.

We can, for instance, easily check if the path is valid using the exists() method.

This returns a Boolean value, that is, either True or False.

# Use the exists() method to check if the path is valid
corpus_dir.exists()

True

We can also check if the path is a directory using the is_dir() method.

# Use the exists() method to check if the path points to a directory
corpus_dir.is_dir()

True

Let’s make sure the path does not point towards a file using the is_file() method.

# Use the exists() method to check if the path points to a file
corpus_dir.is_file()

False

Now that we know that the path points toward a directory, we can use the glob() method to collect all text files in the directory.

glob stands for global, and was first implemented as a program for matching filenames and paths using wildcards.

The glob() method requires one argument, pattern, which takes a string as input. This string defines the kinds of files to be collected. The asterisk symbol * acts as a wildcard, which can refer to any sequence of characters preceding the sequence .txt.

The file identifier .txt is a commonly-used suffix for plain text files.

We also instruct Python to cast the result into a list using the list() function, so we can easily loop over the files in the list.

Finally, we store the result under the variable files and call the result.

# Get all files with the suffix .txt in the directory 'corpus_dir' and cast the result into a list
files = list(corpus_dir.glob(pattern='*.txt'))

# Call the result
files

[PosixPath('data/WP_1990-08-10-25A.txt'),
 PosixPath('data/NYT_1991-01-16-A15.txt'),
 PosixPath('data/WP_1991-01-17-A1B.txt')]

We now have a list of three Path objects that point towards three text files!

This allows us to loop over the files using a for loop and manipulate text in each file.

In the cell below, we iterate over each file defined in the Path object, read and modify its contents, and write them to a new file.

# Loop over the list of Path objects under 'files'. Refer to the individual files using
# the variable 'file'.
for file in files:
    
    # Use the read_text() method of a Path object to read the file contents. Provide 
    # the value 'utf-8' to the 'encoding' argument to declare the file encoding.
    # Store the result under the variable 'text'.
    text = file.read_text(encoding='utf-8')
    
    # Apply the regular expression we defined above to remove excessive punctuation 
    # from the text. Store the result under the variable 'mod_text'
    mod_text = punct.sub('', text)
    
    # Define a new filename which has the prefix 'mod_' by creating a new string. 
    # The Path object contains the filename as a string under the attribute 'name'. 
    # Combine the two strings using the '+' expression.
    new_filename = 'mod_' + file.name
    
    # Define a new Path object that points towards the new file. The Path object 
    # will automatically join the directory and filename for us.
    new_path = Path('data', new_filename)
    
    # Print a status message using string formatting. By adding the prefix 'f' to 
    # a string, we can use curly brackets {} to insert a variable within the string. 
    # Here we add the current file path to the string for printing.
    print(f'Writing modified text to {new_path}')
    
    # Use the write_text() method to write the modified text under 'mod_text' to 
    # the file using UTF-8 encoding.   
    new_path.write_text(mod_text, encoding='utf-8')

Writing modified text to data/mod_WP_1990-08-10-25A.txt
Writing modified text to data/mod_NYT_1991-01-16-A15.txt
Writing modified text to data/mod_WP_1991-01-17-A1B.txt

As you can see from the code block above, Path objects provide two convenient methods for working with text files: read_text() and write_text().

These methods can be used to read and write text from files without using the with statement, which was introduced in the previous section. Just as using the with statement, the file that the Path points to is closed after the text has been read.

This should have given you an idea of the some more powerful methods for manipulating text available in Python, such as regular expressions, and how to apply them to multiple files at the same time.

The following section will teach you how to apply basic natural language processing techniques to texts.

Manipulating text at scale

Contents

Manipulating text at scale¶

Regular expressions¶

Processing multiple files¶