PHP Program to Print All Duplicate Characters in a String

Last Updated : 08 Jul, 2024
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This article will show you how to print duplicate characters in a string using PHP. Detecting and printing duplicate characters in a string is a common task in programming. In this article, we will explore various approaches to achieve this.

Table of Content

Using array_count_values() Function

The array_count_values() function is used to count the occurrences of each value in an array, simplifying the process.

PHP 
<?php 

function printDuplicates($str) { 
	$charCount = array_count_values(str_split($str)); 

	// Print duplicate characters 
	echo "Duplicate characters: "; 
	foreach ($charCount as $char => $count) { 
		if ($count > 1) { 
			echo "$char "; 
		} 
	} 
} 

// Driver code 
$str = "Welcome to GeeksforGeeks"; 

printDuplicates($str); 

?>

Output
Duplicate characters: e o   G k s

Using Associative Array

One of the most basic methods is to use an associative array to keep track of the occurrence of each character in the string.

PHP 
<?php 

function printDuplicates($str) { 
	$charCount = []; 

	// Iterate through each character 
	// in the string 
	for ($i = 0; $i < strlen($str); $i++) { 
		$char = $str[$i]; 

		// Check if the character is already 
		// in the associative array 
		if (isset($charCount[$char])) { 
			$charCount[$char]++; 
		} else { 
			$charCount[$char] = 1; 
		} 
	} 

	// Print duplicate characters 
	echo "Duplicate characters: "; 
	foreach ($charCount as $char => $count) { 
		if ($count > 1) { 
			echo "$char "; 
		} 
	} 
} 

// Driver code 
$str = "Welcome to GeeksforGeeks"; 
printDuplicates($str); 

?>

Output
Duplicate characters: e o   G k s

Using a Regular Expression and preg_match_all()

Another approach to print duplicate characters in a string is by using a regular expression in combination with the preg_match_all() function. This method is particularly useful for efficiently identifying and handling characters within the string.

Example: This approach offers a better way to detect and print duplicate characters in a string, especially when dealing with Unicode characters or requiring efficient pattern matching.

PHP 
<?php
function printDuplicateCharacters($inputString) {
    // Initialize an empty array to store character counts
    $charCounts = [];
    
    // Use preg_match_all to get all characters
    preg_match_all('/./u', $inputString, $matches);
    
    // Iterate through each character and count occurrences
    foreach ($matches[0] as $char) {
        if (isset($charCounts[$char])) {
            $charCounts[$char]++;
        } else {
            $charCounts[$char] = 1;
        }
    }
    
    // Print characters that appear more than once
    foreach ($charCounts as $char => $count) {
        if ($count > 1) {
            echo "Character '$char' appears $count times.\n";
        }
    }
}

// Sample usage
$inputString = "programming";
printDuplicateCharacters($inputString);
?>

Output
Character 'r' appears 2 times.
Character 'g' appears 2 times.
Character 'm' appears 2 times.



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