Last updated: Fri, 02 Jan 2009

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;
?>


To access a local variable within a callback, use currying (delayed argument binding). For example

<?php

function curry($func, $arity) {

    return create_function('', "

        \$args = func_get_args();

        if(count(\$args) >= $arity)

            return call_user_func_array('$func', \$args);

        \$args = var_export(\$args, 1);

        return create_function('','

            \$a = func_get_args();

            \$z = ' . \$args . ';

            \$a = array_merge(\$z,\$a);

            return call_user_func_array(\'$func\', \$a);

        ');

    ");

}



function on_match($transformation, $matches)

{

    return $transformation[strtolower($matches[1])];

}



$transform = array('a' => 'Well,', 'd'=>'whatever', 'b'=>' ');



$callback = curry(on_match, 2);

echo preg_replace_callback('/([a-z])/i', $callback($transform), 'Abcd');



echo "\n";

?>



outputs:



"Well, whatever"



The magic lies in this curry function I found here: http://www.sitepoint.com/forums/showthread.php?threadid=336758

tijn at q-go dot com
05-Jan-2009 04:48


To access a local variable within a callback, use currying (delayed argument binding). For example

<?php

function curry($func, $arity) {

    return create_function('', "

        \$args = func_get_args();

        if(count(\$args) >= $arity)

            return call_user_func_array('$func', \$args);

        \$args = var_export(\$args, 1);

        return create_function('','

            \$a = func_get_args();

            \$z = ' . \$args . ';

            \$a = array_merge(\$z,\$a);

            return call_user_func_array(\'$func\', \$a);

        ');

    ");

}



function on_match($transformation, $matches)

{

    return $transformation[strtolower($matches[1])];

}



$transform = array('a' => 'Well,', 'd'=>'whatever', 'b'=>' ');



$callback = curry(on_match, 2);

echo preg_replace_callback('/([a-z])/i', $callback($transform), 'Abcd');



echo "\n";

?>



outputs:



"Well, whatever"



The magic lies in this curry function I found here: http://www.sitepoint.com/forums/showthread.php?threadid=336758

nicolaspar at gmail dot com
20-Dec-2008 02:33


To spend more than one parameter can do the following (note the "e" parameter in preg_replace function)

<?

$array = array(

1=>'ONE',

2=>'TWO',

3=>'Three'

);



function search(&$array, $str, $foo, $bar){

    return ( empty($array[$str]) ? '['.$foo.'-'.$bar.']' : $array[$str] );

}



function keys(&$array, $str,$foo,$bar){

    return preg_replace('/\[(.*?)\]/e',"search(\$array,$1,\$foo,\$bar)",$str);

}



$str = "One [1] Two [2] Three [3], Other parameter [22]";



echo keys($array, $str,'Foo','Bar');

?>

Nice

harrrrpo at gmail dot com
19-Sep-2008 07:59


The last example -nested BBCode- has a problem and won't work

it should be

<?php

  $regex = '#\[indent]((?:[^\[]|\[(?!/?indent])|(?R))+)\[/indent]#';

?>

([ should be escaped to be \[)



--

a nicer regex i used in a BBCode parser  (BBEngine http://www.phpclasses.org/browse/package/4829.html)

This one Captures Tag Arguments ,Empty tags and 

also uses ?> for performance , wrapped in a simple function for templating (ofcourse it's one line but note's line size is limited :) )

<?php

     /**

     * A Template for the recursive tags matcher RE

     * it generates it for a given tag ,open bracket and closing one

     * $O & $C must be pre-escaped from #'s

     * @param String $tag Tag to be parsed recursively

     * @param String $O   Opeening brackets of tag

     * @param String $C   Closing brackets of tag

     */

    public function Recursive_RE_Generator($tag,$O,$C)

    {

       $re="#{$O}({$tag}.*?){$C}((?>{$O}(?!/?{$tag}[^{$O}]*?{$C})|

[^{$O}]|(?R))*){$O}/{$tag}{$C}#is";

        return $re;

    }

?>

nene at triin dot net
20-May-2008 10:14


The first example is bad, because it creates function for every line it processes. When the file has many lines, you could easily run out of memory. The code should be changed so, that create_function() is used outside of loop.

Sjon at hortensius dot net
24-Jun-2007 11:56


preg_replace_callback returns NULL when pcre.backtrack_limit is reached; this sometimes occurs faster then you might expect. No error is raised either; so don't forget to check for NULL yourself

matt at mattsoft dot net
26-Apr-2006 09:16


it is much better on preformance and better practice to use the preg_replace_callback function instead of preg_replace with the e modifier.



function a($text){return($text);}



// 2.76 seconds to run 50000 times

preg_replace("/\{(.*?)\}/e","a('\\1','\\2','\\3',\$b)",$a);



// 0.97 seconds to run 50000 times

preg_replace_callback("/\{(.*?)\}/s","a",$a);

add a note

preg_replace

preg_quote