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Regex in Python

Module - 8 Python Libraries and Advanced Concepts
Regex in Python

Overview

Regex, or regular expressions, is a ūüí™ powerful tool in Python for manipulating ūüďĚ text. It uses special characters and wildcards to define search patterns and extract information from strings. With its range of flags, ūüďĎ functions, and syntax, regex can solve complex text tasks. Python offers many libraries and tools for regex, making it popular for data science and ūüĆź web development.

Introduction to Regex in Python

Regular expressions (regex) are powerful tools for matching text patterns in Python. Regex can be used to search for specific text strings, replace certain text strings, validate input, and more. Regex is a powerful language that is used to match patterns in strings and text. Provides a concise and flexible means of identifying text strings of interest, such as A specific letter, word, or pattern of letters. Regex is an essential part of any programming language and Python has a strong regex library that is easy to use.

Regular Expression Syntax

import re

pattern = re.compile('\\d+')

result = pattern.findall('The price is $20 and the quantity is 3')

print(result)

# Output: ['20', '3']

The re.compile() function compiles a regular expression pattern into a regular expression object. The regular expression is d+ which matches one or more digits (0-9). The findall() function is, at that point, utilized to search the string for the pattern and return a list of matches. In this illustration, the yield may be a list of strings containing the matched digits '20' and '3'.

Regex Matching in Python

In Python, regex coordinating is done utilizing the re-module. This module gives a number of functions and classes that permit us to explore for designs in a string and manipulate or supplant those patterns when found.

To get begun, we have to import the re-module into our program.

import re

Once the module is imported, you can search for patterns in strings using the re.search() function. This function takes two arguments:

  • The pattern that we are trying to match
  • The string which we are searching

For example, if we wanted to find all instances of the word "cat" in a string, we could use the following code:

my_string = "The cat is in the kitchen"
result = re.search("cat", my_string)

If the pattern is found, the result will be a Match object containing information about the match. Otherwise, the result will be None.

We can also use the re.findall() function to find all pattern occurrences in a string. This function takes two arguments:

  • The pattern that we are trying to match
  • The string which we are searching

For example, if we wanted to find all occurrences of the word "cat" in a string, we could use the following code:

my_string = "The cat is in the kitchen"
results = re.findall("cat", my_string)

The findall() function results will be a list of all matches of the pattern in the string.

Finally, we can use the re. sub() function to replace all occurrences of a pattern in a string. This function takes three arguments:

  • The pattern that we are trying to match
  • The replacement string
  • The string which we are searching

For example, if we wanted to replace all occurrences of the word "cat" in a string with the word "dog", we could use the following code:

my_string = "The cat is in the kitchen"
result = re.sub("cat", "dog", my_string)

The result of the sub() function will be a new string with all the occurrences of the pattern replaced by the new string.

Regex Searching in Python

Regex searching in Python can be done using the re module. This module provides regular expression matching operations similar to those found in Perl. The module can search for patterns in strings, search and replace operations, and split strings into substrings. To use the re-module, the user must first import it into their program.

Example :

import re

# Search for occurances of the pattern 'abc' in the string
string = 'abcdefghijklmnopqrstuvwxyz'
pattern = 'abc'

# Use the 'findall' method to return a list of all matches
matches = re.findall(pattern, string)

# Print the matches
print(matches)

# Output: ['abc']

This code imports the re-module, which provides regular expression comparison operations similar to Perl. It then defines a string and a pattern to search for. The 'findall' method is used to search for all occurrences of the pattern 'abc' in the string and return a list of all matches. Finally, the matches are printed on the console.

Regex Substitution in Python

Regex replacement in Python is done with the re.sub() function. This function takes three parameters: a regular expression pattern, a replacement string, and the string to do the replacement. It then returns a new string with all pattern matches replaced with the replacement string. Example

import re 

text = "This is a test string."

result = re.sub(r"test", "demo", text) 

print(result) 

# Output: This is a demo string.

Grouping and Backreferences in Regex

Grouping and backreferences allow for more complex and powerful regex expressions. The gathering is utilized to group parts of a regex expression to be referenced by a single element, such as a backreference. Backreferences are used to allude to an already matched group inside the same expression. For illustration, in case a regex expression contains two bunches (e.g., w+ and d+), a backreference can allude to the first group when utilized within the second group, permitting the regex expression to match different strings with the same pattern. This can be valuable for matching patterns that can appear at different times in a string, such as a phone or credit card number.

Examples of Regex in Python

#1 Match all alphanumeric characters

import re

pattern = r"[a-zA-Z0-9]"

#2 Match all alphanumeric characters and underscore

import re

pattern = r"[a-zA-Z0-9_]"

This code employments normal expressions to coordinate all alphanumeric characters and underscores. The r"[a-zA-Z0-9_]" tells the regex engine to match any character within the range of a-z, A-Z, 0-9, and the underscore character. The regular expression is put away within the variable pattern.

Conclusion

Regular expressions (regex) in Python may be an effective instrument for pattern matching, text manipulation, and input validation. With regex, designers can effortlessly perform complex text tasks by defining search patterns with uncommon characters and wildcards, making it a basic part of data science and web improvement. Python contains a solid regex library that's simple to utilize and offers a range of functions and classes for regex matching, searching, and substitution.

Key takeaways

  1. Regex may be a capable instrument for text processing and pattern matching.
  2. Python encompasses a module called re, which gives effective regex functions for an assortment of operations.
  3. Regular expressions can be utilized to look for, replace, and extract particular strings from the text.
  4. Regex functions in Python are best utilized when combined with other string functions, such as split(), findall(), and sub().
  5. Python's re-module gives extra highlights, such as case-insensitive matching and greedy/non-greedy matching.
  6. Python's re-module, moreover, gives functions for matching against different patterns at once.
  7. Regex can be more capable by utilizing extraordinary characters, such as groupings and character classes.
  8. Regex can be utilized to approve user input, such as email addresses and phone numbers.

Quiz

  1. What symbol is used to indicate the start of a regular expression in Python?
    1. * 
    2. & 
    3. /

Answer:a. *

  1. What is the purpose of the re.search() function in Python?
    1. To find all matches in a string 
    2. To split a string into a list 
    3. To find the first match in a string 
    4. To replace a substring in a string

Answer:c. To find the first match in a string

  1. What is the output of the following code?
import re
string = "Python is a great language"

pattern = r"great"

print(re.findall(pattern, string))

a. ["great"] 

b. ["Python", "great", "language"] 

c. ["Python", "is", "a", "great", "language"] 

d. [5]

Answer:a. ["great"]

  1. In Python, what is the distinction between the re.match() and re.search() functions?
    1. re.match() looks for a pattern at the starting of a string, whereas re.search() looks for a pattern anywhere within the string. 
    2. re.match() looks for a pattern anywhere within the string, whereas re.search() looks for a pattern at the starting of a string. 
    3. re.match() looks for a pattern that matches the whole string, whereas re.search() looks for a pattern that partially matches the string. 
    4. re.match() looks for a pattern that partially matches the string, whereas re.search() looks for a pattern that matches the complete string.

Answer:b. re.match() looks for a pattern anywhere within the string, whereas re.search() looks for a pattern at the starting of a string.

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