如何在递归正则表达式中反向引用匹配？

<?php
$data = 'id=1

username=foobar

comment=This is

a sample

comment';

// If you are at PHP >= 5.3.0 (using preg_replace_callback)
$result = preg_replace_callback(
    '/b(comment=)(.+)$/ms',
    function (array $matches) {
        return $matches[1] . preg_replace("/[rn]+/", " ", $matches[2]);
    },
    $data
);

// If you are at PHP < 5.3.0 (using preg_replace with e modifier)
$result = preg_replace(
    '/b(comment=)(.+)$/mse',
    '"1" . preg_replace("/[rn]+/", " ", "2")',
    $data
);

var_dump($result);

string(59) "id=1

username=foobar

comment=This is a sample comment"