How to Get All Substrings of a Given String in PHP ?

Extracting all possible substrings from a given string is a common task in text processing, pattern recognition, and various other applications in PHP programming. This operation involves generating every possible combination of characters present in the string, including both contiguous and non-contiguous sequences.

A substring is any continuous sequence of characters within a string. For a string of length n, the number of possible substrings is n(n + 1)/2, not counting the empty string. This concept is crucial in many domains, including search operations, data analysis, and when implementing algorithms that process text data.

Here are some common approaches:

Approach 1: Nested Loops for Contiguous Substrings

The basic method to get all contiguous substrings of a string is by using nested loops. The outer loop iterates through each character in the string as a starting point, and the inner loop generates substrings starting from the outer loop's current position.

<?php 

function getAllSubstrings($string) { 
	$substrings = []; 
	$length = strlen($string); 
	for ($i = 0; $i < $length; $i++) { 
		for ($j = $i; $j < $length; $j++) { 
			$substrings[] = substr($string, $i, $j - $i + 1); 
		} 
	} 
	return $substrings; 
} 

// Driver code 
$string = "abc"; 
$substrings = getAllSubstrings($string); 
echo "All substrings of '$string':\n"; 
print_r($substrings); 

?>

All substrings of 'abc':
Array
(
    [0] => a
    [1] => ab
    [2] => abc
    [3] => b
    [4] => bc
    [5] => c
)

Approach 2: Bitmasking for All Subsequences

To include both contiguous and non-contiguous substrings (subsequences), you can use a more advanced technique involving bitmasking. This method represents each substring as a binary number, where each bit determines whether a character at a certain position is included in the substring.

<?php 

function getAllSubstrings($string) { 
	$subsequences = []; 
	$length = strlen($string); 
	$total = 1 << $length; 

	for ($i = 0; $i < $total; $i++) { 
		$subseq = ''; 
		for ($j = 0; $j < $length; $j++) { 
			if ($i & (1 << $j)) { 
				$subseq .= $string[$j]; 
			} 
		} 
		if (!empty($subseq)) { 
			$subsequences[] = $subseq; 
		} 
	} 
	return $subsequences; 
} 

// Driver code 
$string = "abc"; 
$substrings = getAllSubstrings($string); 
echo "All substrings of '$string':\n"; 
print_r($substrings); 

?>

All substrings of 'abc':
Array
(
    [0] => a
    [1] => b
    [2] => ab
    [3] => c
    [4] => ac
    [5] => bc
    [6] => abc
)

Approach 3: Using Recursion for Non-Contiguous Substrings

Another method to generate all possible substrings, including both contiguous and non-contiguous sequences, is through recursion. This approach involves recursively including or excluding each character in the string to form substrings.

Example: This recursive approach is elegant and straightforward, especially for generating all possible combinations of substrings, including non-contiguous ones, making it suitable for various text processing and algorithmic tasks.

<?php
function generateSubstrings($str)
{
    $result = [];
    $current = "";
    generateSubstringsRecursively($str, $current, 0, $result);
    return $result;
}

function generateSubstringsRecursively($str, $current, $index, &$result)
{
    if ($index == strlen($str)) {
        if ($current != "") {
            $result[] = $current;
        }
        return;
    }

    // Include the current character
    generateSubstringsRecursively(
        $str,
        $current . $str[$index],
        $index + 1,
        $result
    );

    // Exclude the current character
    generateSubstringsRecursively($str, $current, $index + 1, $result);
}

$str = "abc";
$substrings = generateSubstrings($str);
print_r($substrings);

?>

Array
(
    [0] => abc
    [1] => ab
    [2] => ac
    [3] => a
    [4] => bc
    [5] => b
    [6] => c
)