preg_replace_callback

Perform a regular expression search and replace using a callback

&reftitle.description; `mixed` `preg_replace_callback` `mixed` `pattern` `callable` `callback` `mixed` `subject` `int` `limit`-1 `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`.

&reftitle.parameters;

`pattern`

- The pattern to search for. It can be either a string or an array with strings.

`callback`

- A callback that will be called and passed an array of matched elements in the `subject` string. The callback should return the replacement string. This is the callback signature:

  `string```handler`` `array` `matches`

  You'll often need the `callback` function for a `preg_replace_callback` in just one place. In this case you can use an anonymous function to declare the 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 function's name not used anywhere else.

  ### `preg_replace_callback` and anonymous function

      <?php
      /* 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);
          $line = preg_replace_callback(
              '|<p>\s*\w|',
              function ($matches) {
                  return strtolower($matches[0]);
              },
              $line
          );
          echo $line;
      }
      fclose($fp);
      ?>

`subject`

- The string or an array with strings to search and replace.

`limit`

- The maximum possible replacements for each pattern in each `subject` string. Defaults to `-1` (no limit).

`count`

- If specified, this variable will be filled with the number of replacements done.

&reftitle.returnvalues;

`preg_replace_callback` returns an array if the `subject` parameter is an array, or a string otherwise. On errors the return value is &null;

If matches are found, the new subject will be returned, otherwise `subject` will be returned unchanged.

&reftitle.changelog;

&Version; | &Description;
5.1.0 | The `count` parameter was added

&reftitle.examples;

### `preg_replace_callback` example

    <?php
    // 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(
                "|(\d{2}/\d{2}/)(\d{4})|",
                "next_year",
                $text);

    ?>

&example.outputs;

    April fools day is 04/01/2003
    Last christmas was 12/24/2002

### `preg_replace_callback` using recursive structure to handle encapsulated BB code

    <?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;
    ?>

&reftitle.seealso;

- PCRE Patterns
- `preg_replace`
- `preg_last_error`
- Anonymous functions
- &seealso.callback;