Understanding PHP preg_filter(): Syntax, Parameters, and Practical Example

The preg_filter() function in PHP executes a regular‑expression search and replace, but unlike preg_replace() it returns only the parts of the subject that match the pattern (after any replacements).

Its signature is preg_filter(mixed $pattern, mixed $replacement, mixed $subject, int $limit = -1, int &$count = null) . The function behaves the same as preg_replace() except for the return value.

Parameters :

pattern : The regex pattern(s) to search for. Can be a string or an array of strings, with optional PCRE modifiers.

replacement : The replacement string(s). Supports back‑references like \1 or $1 . If arrays are used, each pattern is paired with the corresponding replacement.

subject : The string or array of strings to be processed.

limit : Maximum number of replacements per pattern (default -1 for unlimited).

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, an empty array (for array subjects) or NULL (for string subjects) is returned.

Example :

Output :

preg_filter returns
Array
(
    [0] => A:C:1
    [1] => B:C:a
    [2] => A:2
    [3] => B:3
    [4] => A:4
)

preg_replace returns
Array
(
    [0] => A:C:1
    [1] => B:C:a
    [2] => A:2
    [3] => B:3
    [4] => A:4
)

The example demonstrates that preg_filter() returns only the elements where a pattern matched, while preg_replace() returns the full subject array with replacements applied.