Understanding PHP preg_filter: Syntax, Parameters, and Examples

preg_filter() is a PHP function that performs a regular‑expression search and replace, returning only the parts of the subject that match the pattern.

It is equivalent to preg_replace() except that it returns only the transformed matches; non‑matching elements are omitted. The function signature is

preg_filter(mixed $pattern, mixed $replacement, mixed $subject, int $limit = -1, int &$count = null)

.

Parameters

pattern : the regex pattern(s) to search for (string or array).

replacement : the replacement string(s) (string or array).

subject : the input string or array to be processed.

limit : maximum number of replacements per pattern; default -1 (no limit).

count : optional variable that receives the total number of replacements performed.

Return value

If $subject is an array, an array of matched results is returned; otherwise a string is returned. If no matches are found or an error occurs, NULL (or an empty array for array subjects) is returned.

Example 1

<?php
$subject = array('1','a','2','b','3','A','4');
$pattern = array('/\d/', '/[a-z]/', '/[1a]/');
$replace = array('A:$0','B:$0','C:$0');

echo "preg_filter 返回值:
";
print_r(preg_filter($pattern, $replace, $subject));

echo "preg_replace 返回值:
";
print_r(preg_replace($pattern, $replace, $subject));
?>

The output shows that preg_filter returns only the elements that matched the patterns, while preg_replace returns the full array with non‑matching elements unchanged.

Example 2

<?php
$subject = array('1','a','2','b','3','A','4');
$result = preg_filter('/\d/', '$0', $subject);
print_r($result);
?>

This example demonstrates a simple use of preg_filter to extract numeric characters from an array.

In summary, preg_filter is useful when you need only the matched portions of a string or array after applying regular‑expression replacements.