Python Regular Expressions

Introduction

Regular expressions, also known as "regex," are a powerful tool for working with strings. They can be used to search, edit, and manipulate text in a variety of ways. In this lab, you will learn the basics of working with regular expressions in Python.

Achievements

• Regular Expressions

Skills Graph

Import the Re Module

To use regular expressions in Python, you will need to import the re module.

Open up a new Python interpreter session and type the following code:

python3

Add the following line at the top of your Python script:

import re

Compile a Regular Expression

Before you can use a regular expression to search for a pattern in a string, you must first compile it. To compile a regular expression, you can use the re.compile() function.

For example, the following code will compile a regular expression that searches for the letter "a" in a string:

regex = re.compile(r"a")

Note that the r before the string indicates that it is a "raw" string, which means that backslashes are treated as literal characters rather than escape characters. This is generally recommended when working with regular expressions to avoid confusion.

Use the Search() Method

Now that you have compiled a regular expression, you can use the search() method to search for a pattern in a string.

For example, the following code will search for the letter "a" in the string "cat":

match = regex.search("cat")
print(match) ## Output: <re.Match object; span=(1, 2), match='a'>

The search() method will return a Match object if the pattern is found, or None if it is not found.

Use the Match() Method

The match() method is similar to the search() method, but it only matches patterns at the beginning of a string.

For example, the following code will match the pattern "cat" at the beginning of the string "catdog":

regex = re.compile(r"cat")
match = regex.match("catdog")
print(match) ## Output: <re.Match object; span=(0, 3), match='cat'>

Use the Findall() Method

The findall() method returns a list of all the instances of a pattern in a string.

For example, the following code will find all the letters "a" in the string "cat":

regex = re.compile(r"a")
matches = regex.findall("cat")
print(matches)  ## Output: ["a"]

There are many special characters that you can use in regular expressions to match more complex patterns. Here are a few examples:

• . (dot) matches any single character
• * matches zero or more of the preceding character
• + matches one or more of the preceding character
• ? matches zero or one of the preceding character
• [characters] matches any one of the characters inside the square brackets

Use Groups

You can use groups in your regular expressions to extract specific parts of a pattern. Groups are created by enclosing a part of the pattern in parentheses.

For example, the following regular expression will match any word that starts with "cat" and ends with "dog":

regex = re.compile(r"cat(.*)dog")

You can then use the group() method of the Match object to extract the part of the pattern that was matched by the group. For example:

match = regex.search("catdog")
print(match.group(1))  ## Output: "" (empty string)

match = regex.search("catfooddog")
print(match.group(1))  ## Output: "food"

Use the Sub() Method

The sub() method allows you to replace all instances of a pattern in a string with a different string.

For example, the following code will replace all the letters "a" in the string "cat" with the letter "b":

regex = re.compile(r"a")
new_string = regex.sub("b", "cat")
print(new_string)  ## Output: "cbt"

Use the Split() Method

The split() method allows you to split a string by a pattern.

For example, the following code will split the string "cat dog" by the space character:

regex = re.compile(r" ")
words = regex.split("cat dog")
print(words)  ## Output: ["cat", "dog"]

Regex101

Regex101 is an online regular expression testing tool that allows you to test your regular expressions against a string of your choice. It is a useful resource for debugging and developing regular expressions, as it provides detailed information about the matches and errors in your regex.

• https://regex101.com

To use Regex101, you can enter a regular expression in the "Regex" field, and a string to test it against in the "Test String" field. As you type, the tool will highlight any matches in the string, and it will also display any errors or warnings in the "Result" section.

In addition to testing regular expressions, Regex101 also includes a number of useful features, such as the ability to save and share your regular expressions, a library of common regular expression patterns, and options for customizing the way the tool performs matches.

Overall, Regex101 is a valuable tool for anyone working with regular expressions in Python or any other programming language.

Summary

In this lab, you learned how to use regular expressions in Python to search, match, and manipulate strings. You learned how to compile regular expressions, how to use various methods such as search(), match(), findall(), and sub(), and how to use special characters and groups to match more complex patterns.

I hope this helps! Let me know if you have any questions.