Minifying final HTML output using regular expressions with CodeIgniter

Question

Google pages suggest you to minify HTML, that is, remove all the unnecessary spaces. CodeIgniter does have the feature of giziping output or it can be done via .htacce

Answer 1

I implemented the answer from @ridgerunner in two projects, and ended up hitting some severe slowdowns (10-30 second request times) in staging for one of the projects. I found out that I had to set both pcre.recursion_limit and pcre.backtrack_limit quite low for it to even work, but even then it would give up after about 2 senconds of processing and return false.

Since that, I've replaced it with this solution (with easier-to-grasp regex), which is inspired by the outputfilter.trimwhitespace function from Smarty 2. It does no backtracking or recursion, and works every time (instead of catastrophically failing once in a blue moon):

function filterHtml($input) {
    // Remove HTML comments, but not SSI
    $input = preg_replace('//s', '', $input);

    // The content inside these tags will be spared:
    $doNotCompressTags = ['script', 'pre', 'textarea'];
    $matches = [];

    foreach ($doNotCompressTags as $tag) {
        $regex = "!<{$tag}[^>]*?>.*?!is";

        // It is assumed that this placeholder could not appear organically in your
        // output. If it can, you may have an XSS problem.
        $placeholder = "@@<'-placeholder-$tag'>@@";

        // Replace all the tags (including their content) with a placeholder, and keep their contents for later.
        $input = preg_replace_callback(
            $regex,
            function ($match) use ($tag, &$matches, $placeholder) {
                $matches[$tag][] = $match[0];
                return $placeholder;
            },
            $input
        );
    }

    // Remove whitespace (spaces, newlines and tabs)
    $input = trim(preg_replace('/[ \n\t]+/m', ' ', $input));

    // Iterate the blocks we replaced with placeholders beforehand, and replace the placeholders
    // with the original content.
    foreach ($matches as $tag => $blocks) {
        $placeholder = "@@<'-placeholder-$tag'>@@";
        $placeholderLength = strlen($placeholder);
        $position = 0;

        foreach ($blocks as $block) {
            $position = strpos($input, $placeholder, $position);
            if ($position === false) {
                throw new \RuntimeException("Found too many placeholders of type $tag in input string");
            }
            $input = substr_replace($input, $block, $position, $placeholderLength);
        }
    }

    return $input;
}