# preg_replace_callback

(PHP 4 >= 4.0.5, PHP 5)

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

### Description

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` .

### 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.

  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 function's name not used anywhere else.

  Example#1 preg_replace_callback() and create_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|',
  create_function(
  // single quotes are essential here,
  // or alternative escape all $ as \$
  '$matches',
  'return strtolower($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.

### Return Values

preg_replace_callback() returns an array if the `subject` parameter is an array, or a string otherwise.

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

### ChangeLog

Version | Description
--- | ---
5.1.0 | The `count` 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

- preg_replace()
- create_function()
- information about the callback type