PHP preg_replace_callback function

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preg_replace_callback

(PHP 4 >= 4.0.5, PHP 5)

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.

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

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()

/* 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);

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

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

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

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.

Example 2. 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(
            "|(\d{2}/\d{2}/)(\d{4})|",
            "next_year",
            $text);

The above example will output:

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;