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# preg_replace_callback

(PHP 4 >= 4.0.5)

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])

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` that will be called and passed an array of matched elements in the subject string. The callback should return the replacement string.

Exemplo 1. 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);
// result is:
// April fools day is 04/01/2003
// Last christmas was 12/24/2002
?>`

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 functions name not used anywhere else.

Exemplo 2. 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);
?>`

See also preg_replace() and create_function().

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