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Last updated: Tue, 19 Sep 2006


(PHP 4 >= 4.0.5, PHP 5)

preg_replace_callback -- Perform a regular expression search and replace using a callback


mixed preg_replace_callback ( mixed pattern, callback callback, mixed subject [, int limit [, int &count]] )

The behavior of this function is almost identical to preg_replace(), except for the fact that instead of replacement parameter, one should specify a callback that will be called and passed an array of matched elements in the subject string. The callback should return the replacement string. See preg_replace() for description of other parameters.

Example 1. preg_replace_callback() example

// this text was used in 2002
// we want to get this up to date for 2003
$text = "April fools day is 04/01/2002\n";
$text.= "Last christmas was 12/24/2001\n";
// the callback function
function next_year($matches) 
  // as usual: $matches[0] is the complete match
  // $matches[1] the match for the first subpattern
  // enclosed in '(...)' and so on
  return $matches[1].($matches[2]+1);
echo preg_replace_callback(
// result is:
// April fools day is 04/01/2003
// Last christmas was 12/24/2002

You'll often need the callback function for a preg_replace_callback() in just one place. In this case you can use create_function() to declare an anonymous function as callback within the call to preg_replace_callback(). By doing it this way you have all information for the call in one place and do not clutter the function namespace with a callback functions name not used anywhere else.

Example 2. preg_replace_callback() and create_function()

/* a unix-style command line filter to convert uppercase
 * letters at the beginning of paragraphs to lowercase */
$fp = fopen("php://stdin", "r") or die("can't read stdin");
while (!feof($fp)) {
    $line = fgets($fp);
    $line = preg_replace_callback(
            // single quotes are essential here,
            // or alternative escape all $ as \$
            'return strtolower($matches[0]);'
    echo $line;

Example 3. preg_replace_callback() using recursive structure to handle encapsulated BB code

$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;

Note: count parameter is available since PHP 5.1.0.

See also preg_replace(), create_function(), and information about the callback type.

Last updated: Tue, 19 Sep 2006