How to Use preg_match_all in PHP: Syntax and Examples

Use preg_match_all in PHP to find all matches of a regular expression pattern in a string. It returns the number of matches found and fills an array with all matched results. The function signature is preg_match_all(string $pattern, string $subject, array &$matches).

Syntax

The preg_match_all function searches a string for all matches of a regular expression pattern. It takes three main parameters:

• $pattern: The regular expression pattern to search for, enclosed in delimiters like /pattern/.
• $subject: The string to search within.
• &$matches: A variable passed by reference that will hold all matches found.

The function returns the number of matches found or 0 if none are found.

php

int preg_match_all(string $pattern, string $subject, array &$matches)

Example

This example finds all words starting with the letter 'a' in a sentence and prints them.

php

<?php
$subject = "An apple a day keeps the doctor away.";
$pattern = '/\ba\w*/i'; // \b means word boundary, i means case-insensitive

if (preg_match_all($pattern, $subject, $matches)) {
    echo "Found matches:\n";
    print_r($matches[0]);
} else {
    echo "No matches found.";
}
?>

Output

Found matches:
Array
(
    [0] => An
    [1] => apple
    [2] => a
    [3] => away
)

Common Pitfalls

Common mistakes when using preg_match_all include:

• Forgetting to pass the $matches array by reference, which is required to store results.
• Using incorrect delimiters or forgetting to escape special characters in the pattern.
• Confusing preg_match (which finds only the first match) with preg_match_all (which finds all matches).

php

<?php
// Wrong: missing & before $matches, so $matches stays empty
$pattern = '/\d+/';
$subject = '123 456 789';
$matches = [];
preg_match_all($pattern, $subject, $matches); // $matches will be filled correctly

// Right: pass $matches by reference
preg_match_all($pattern, $subject, $matches);
print_r($matches[0]);
?>

Output

Array
(
    [0] => 123
    [1] => 456
    [2] => 789
)

Quick Reference

Parameter	Description
$pattern	Regular expression pattern with delimiters
$subject	Input string to search
&$matches	Array to store all matches found
Return value	Number of matches found or 0 if none

Key Takeaways

Use preg_match_all to find all matches of a regex pattern in a string.

Always pass the matches array by reference to collect results.

Remember preg_match_all returns the count of matches found.

Use proper regex delimiters and escape special characters.

preg_match_all differs from preg_match by returning all matches, not just the first.