Filter PHP Array with Regex Using preg_grep

This article explains how to use the built-in preg_grep() function in PHP to filter array elements using regular expressions. You will learn the syntax of the function, explore practical code examples for matching and excluding patterns, and understand how to handle array keys during the filtering process.

What is preg_grep()?

The preg_grep() function is a built-in PHP function used to search an array for elements that match a specific regular expression. It returns a new array containing only the elements that match (or do not match, depending on your settings) the given pattern.

Syntax

preg_grep(string $pattern, array $array, int $flags = 0): array

• $pattern: The regular expression pattern to search for, as a string.
• $array: The input array containing the strings you want to filter.
• $flags: Optional. If set to PREG_GREP_INVERT, the function returns array elements that do not match the pattern.

Practical Examples

1. Basic Matching (Finding Matches)

By default, preg_grep() returns all elements that match the regular expression. In this example, we will filter an array to find elements that start with the letters “ap”.

<?php
$input = ["apple", "banana", "apricot", "cherry", "grape"];

// Regex to match words starting with "ap"
$pattern = "/^ap/"; 

$result = preg_grep($pattern, $input);

print_r($result);
?>

Output:

Array
(
    [0] => apple
    [2] => apricot
)

2. Inverted Matching (Excluding Matches)

If you want to filter out elements that match a pattern, use the PREG_GREP_INVERT flag. This returns all elements that do not match the regular expression.

<?php
$input = ["apple", "banana", "apricot", "cherry", "grape"];

// Regex to match words starting with "ap"
$pattern = "/^ap/"; 

// Invert the match to exclude words starting with "ap"
$result = preg_grep($pattern, $input, PREG_GREP_INVERT);

print_r($result);
?>

Output:

Array
(
    [1] => banana
    [3] => cherry
    [4] => grape
)

Handling Array Keys

As shown in the outputs above, preg_grep() preserves the original keys of the input array. If you need to reset the array keys numerically (starting from 0), pass the result through the array_values() function.

<?php
$input = ["apple", "banana", "apricot", "cherry"];
$pattern = "/^ap/";

$filtered = preg_grep($pattern, $input);

// Reset keys
$reindexed = array_values($filtered);

print_r($reindexed);
?>

Output:

Array
(
    [0] => apple
    [1] => apricot
)