Capturing groups and backreferences in PHP - Mini Project: Build & Apply

Capturing groups and backreferences

📖 Scenario: You are working on a PHP script that processes text to find repeated words. This is useful for cleaning up typos or emphasizing repeated words in a sentence.

🎯 Goal: Build a PHP script that uses a regular expression with capturing groups and backreferences to find repeated words in a string.

📋 What You'll Learn

Create a string variable with a sentence containing repeated words.

Create a regular expression pattern using capturing groups to detect repeated words.

Use the pattern with preg_match_all to find all repeated words.

Print the repeated words found.

💡 Why This Matters

🌍 Real World

Detecting repeated words helps clean up text input, improve readability, and catch typing errors automatically.

💼 Career

Understanding capturing groups and backreferences is essential for text processing, data validation, and building search or cleanup tools in PHP.

1

Create the input string

Create a string variable called text with the value "This is is a test test sentence.".

PHP

# Create a string variable called text with the value "This is is a test test sentence."
# Your code here

Need a hint?

Use double quotes for the string and assign it to $text.

2

Create the regex pattern with capturing groups

Create a variable called pattern and assign it the regex string "/\b(\w+) \1\b/" which uses a capturing group and a backreference to find repeated words.

PHP

<?php
$text = "This is is a test test sentence.";
# Create a variable called pattern with the regex string "/\b(\w+) \1\b/"
# Your code here

Need a hint?

The pattern uses \b for word boundaries, (\w+) to capture a word, and \1 to match the same word again.

3

Find all repeated words using preg_match_all

Use preg_match_all with $pattern and $text to find all matches. Store the matches in a variable called matches.

PHP

<?php
$text = "This is is a test test sentence.";
$pattern = "/\b(\w+) \1\b/";
# Use preg_match_all with $pattern and $text to find matches and store in $matches
# Your code here

Need a hint?

Use preg_match_all function with the pattern and text variables, and pass $matches as the third argument to collect results.

4

Print the repeated words found

Print the repeated words found by accessing $matches[0] using print_r.

PHP

<?php
$text = "This is is a test test sentence.";
$pattern = "/\b(\w+) \1\b/";
preg_match_all($pattern, $text, $matches);
# Print the repeated words found using print_r($matches[0])
# Your code here

Need a hint?

Use print_r($matches[0]) to display the array of repeated words.