If you have a huge page which contains lot of HTML tags, probably your site will be slow, throw error for pcre.backtrack_limit or hang. These problems come from the following regexp which has serious impact on performance:

// Replace all unknown protocols in CSS background image.
if (strpos($buffer, 'style=') !== false)
{
	$regex_url  = '\s*url\s*\(([\'\"]|\&\#0?3[49];)?(?!/|\&\#0?3[49];|' . $protocols . '|\#)([^\)\'\"]+)([\'\"]|\&\#0?3[49];)?\)';
	$regex  = '#style=\s*([\'\"])(.*):' . $regex_url . '#m';
	$buffer = preg_replace($regex, 'style=$1$2: url($3' . $base . '$4$5)', $buffer);
	$this->checkBuffer($buffer);
}

The description of this part is not so good, I think. In real this part fixes the relative url's in the style attributes url(). Example: http://codepad.org/bIUgVuSw

On my test system, ~1000 div needed 1sec to run this regexp.

Test code

<?php

$protocols = '[a-zA-Z0-9\-]+:';

$regex_url = '\s*url\s*\(([\'\"]|\&\#0?3[49];)?(?!/|\&\#0?3[49];|' . $protocols . '|\#)([^\)\'\"]+)([\'\"]|\&\#0?3[49];)?\)';
$regex     = '#style=\s*([\'\"])(.*):' . $regex_url . '#m';

$base = 'https://mybase.com/';

$buffer = '';

for ($i = 0; $i < 1030; $i++) {
    $buffer .= '<div style="background-image: url(\'http://test.com/picture.jpg\')">' . $i . '</div>';
}

$buffer = preg_replace($regex, 'style=$1$2: url($3' . $base . '$4$5)', $buffer);

echo $buffer;

Debug

The problem happens only when the style attribute does not have anything to match. I tries to combine non matched string parts to find a possible match.

Possible solution for discussion

In the original version, the regexp matches everything until it finds a : character. So instead using a plain wildcard, it would be better to match every character until a HTML tag close character found [^>].

Original:
$regex = '#style=\s*([\'\"])(.*):' . $regex_url . '#m';
Updated:
$regex = '#style=\s*(['"])([^>]*):' . $regex_url . '#m';

Test code

<?php
$protocols = '[a-zA-Z0-9\-]+:';

$regex_url = '\s*url\s*\(([\'\"]|\&\#0?3[49];)?(?!/|\&\#0?3[49];|' . $protocols . '|\#)([^\)\'\"]+)([\'\"]|\&\#0?3[49];)?\)';
$regex     = '#style=\s*([\'\"])([^>]*):' . $regex_url . '#m';

$base = 'https://mybase.com/';

$buffer = '';

for ($i = 0; $i < 4000; $i++) {
    $buffer .= '<div style="background-image: url(\'/picture.jpg\');">' . $i . '</div>';
}

echo preg_replace($regex, 'style=$1$2: url($3' . $base . '$4$5)', $buffer);