Using preg_replace_callback in PHP for Regex Replacement with Callbacks

preg_replace_callback() performs a regular‑expression search and replaces matches using a user‑defined callback function, behaving like preg_replace() except that the replacement is generated by the callback.

Parameters: pattern (string or array to search), callback (callable that receives matches and returns the replacement string), subject (string or array to be processed), limit (maximum number of replacements per pattern, default -1 for unlimited), and count (optional variable that receives the number of replacements performed).

Example 1 reads lines from STDIN and converts the first character of each line to lowercase using an anonymous function as the callback.

<?php
$fp = fopen("php://stdin", "r") or die("can't read stdin");
while (!feof($fp)) {
    $line = fgets($fp);
    $line = preg_replace_callback('|<p>\s*\w|', function($matches) {
        return strtolower($matches[0]);
    }, $line);
    echo $line;
}
fclose($fp);
?>

Example 2 shows how to increment the year part of dates in a text by one using a named callback function next_year that adds one to the captured year group.

<?php
$text = "April fools day is 04/01/2020
";
$text = "Last christmas was 12/24/2020
";
function next_year($matches) {
    // $matches[0] is the full match, $matches[1] is the month/day, $matches[2] is the year
    return $matches[1] . ($matches[2] + 1);
}
echo preg_replace_callback("|(\d{2}/\d{2}/)(\d{4})|", "next_year", $text);
?>

Example 3 demonstrates a recursive parsing of custom [indent]…[/indent] tags, converting them into nested <div> elements with increased left margin, using preg_replace_callback() together with a recursive function.

<?php
$input = "plain [indent] deep [indent] deeper [/indent] deep [/indent] plain";
function parseTagsRecursive($input) {
    $regex = '#\[indent]((?:[^[]|\[(?!/?indent])|(?R))+?)\[/indent]#';
    if (is_array($input)) {
        $input = '<div style="margin-left:10px">' . $input[1] . '</div>';
    }
    return preg_replace_callback($regex, 'parseTagsRecursive', $input);
}
$output = parseTagsRecursive($input);
echo $output;
?>