Perform a regular expression search and replace using a callback

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- PHP Manual
- PCRE Functions
- Perform a regular expression search and replace using a callback

# preg_replace_callback

(PHP 4 >= 4.0.5, PHP 5, PHP 7, PHP 8)

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

### Description

preg_replace_callback(
string|array `$pattern`,
callable `$callback`,
string|array `$subject`,
int `$limit` = -1,
int `&$count` = `null`,
int `$flags` = 0
): string|array|null

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`.

### 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:

  handler(array `$matches`): string

  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.

  Example #1 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.

`flags`

  `flags` can be a combination of the `PREG_OFFSET_CAPTURE` and `PREG_UNMATCHED_AS_NULL` flags, which influence the format of the matches array. See the description in preg_match() for more details.

### Return Values

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.

### Errors/Exceptions

If the regex pattern passed does not compile to a valid regex, an `E_WARNING` is emitted.

### Changelog

Version | Description
--- | ---
7.4.0 | The `flags` parameter was added.

### Examples

Example #2 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);

    ?>

The above example will output:

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

Example #3 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;
    ?>

### See Also

- PCRE Patterns
- preg_replace_callback_array() - Perform a regular expression search and replace using callbacks
- preg_quote() - Quote regular expression characters
- preg_replace() - Perform a regular expression search and replace
- preg_last_error() - Returns the error code of the last PCRE regex execution
- Anonymous functions