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Backreferences

Backreferences in regular expressions allow you to refer to a previously matched group in the pattern. They are useful for cases where you need to match a pattern that repeats a previously matched substring, such as matching repeated words or character sequences.

To create a backreference, you use the backslash followed by the group number, \n, where n is the number of the group you want to refer to. For example, the backreference \1 refers to the first group, \2 refers to the second group, and so on.

Here's an example of using a backreference to match repeated words:

import re
 
text = "The cat in the hat hat"
pattern = r"\b(\w+)\b\s+\1"
 
matches = re.findall(pattern, text)
 
print(matches)

In this example, the pattern \b(\w+)\b\s+\1 matches a word boundary, followed by one or more word characters (which are captured into the first group), followed by one or more whitespace characters, followed by the backreference \1, which matches the exact same string as the first captured group. The findall() function returns a list of all the matched groups.

The output of this program is:

['hat']

As you can see, the findall() function has returned a list containing the matched group "hat", which is repeated twice in the input string.

Backreferences can also be used in replacement strings when using the sub() function to replace matched patterns with new strings. For example, the following code replaces all occurrences of "dog" followed by "cat" with "catdog":

import re
 
text = "The dog chased the cat and the cat chased the dog"
pattern = r"dog\s+cat"
 
new_text = re.sub(pattern, "catdog", text)
 
print(new_text)

In this example, the pattern dog\s+cat matches the substring "dog" followed by one or more whitespace characters, followed by the substring "cat". The sub() function replaces all occurrences of this pattern with the string "catdog". Since there are no capturing groups in the pattern, we don't need to use backreferences in the replacement string.

The output of this program is:

The catdog chased the cat and the cat chased the catdog 
As you can see, the sub() function has replaced both occurrences of "dog cat" with "catdog".

Exploring the basics of string manipulation Combining strings using concatenation Accessing elements using indexing in Python Slicing data for efficient subsetting Controlling data sampling with stride Performing operations on strings in Python Changing letter cases in Python Breaking strings into parts using splitting Combining strings using join operation Removing leading/trailing characters in Python Searching and replacing text in Python Searching for substrings in Python How to use the index function in Python Counting occurrences of a character in a string using Python Replacing text in Python using replace() function Formatting strings using positional placeholders Formatting strings in Python Formatting methods in python Formatting strings with positional placeholders Rearranging values in Python Using named placeholders for string formatting Applying format specifiers for precise data formatting Customizing datetime output using formatting Creating dynamic strings with f-strings in Python Converting data types in Python Accessing elements using index lookups in Python Understanding and using escape sequences in Python Performing inline operations in Python Invoking functions in Python Implementing the template method pattern in Python Performing value substitution in Python Exploring regular expressions with the re module in Python Metacharacters supported by the re module in python Repeating characters using loops in python Using quantifiers to match patterns in python with the re module Regular expression metacharacters in python Using special characters in python Using the OR operator in the re module in python Using the OR operand in regular expressions with the re module Understanding the difference between greedy and nongreedy matching in regular expressions Utilizing grouping and capturing in regular expressions with the re module Utilizing the pipe operator in regular expressions with the re module Understanding non-capturing groups in regular expressions Utilizing backreferences in regular expressions for pattern matching Understanding numbered groups in regular expressions Utilizing named groups in regular expressions for pattern matching Understanding lookaround assertions in regular expressions Understanding look-ahead assertions in regular expressions Understanding positive look-ahead assertions in regular expressions Understanding negative look-ahead assertions in regular expressions Understanding look-behind assertions in regular expressions Understanding positive look-behind assertions in regular expressions Understanding negative look-behind assertions in regular expressions