An exemple :
<?php
$a = new ArrayIterator(array('test1', 'test2', 'test3'));
$i = new RegexIterator($a, '/^(test)(\d+)/', RegexIterator::REPLACE);
$i->replacement = '$2:$1';
print_r(iterator_to_array($i));
/*
Array
(
[0] => 1:test
[1] => 2:test
[2] => 3:test
)
*/
?>