Using regular expressions to extract the first image source from html codes?

Question

I would like to know how this can be achieved.

Assume: That there\'s a lot of html code containing tables, divs, images, etc.

Problem: How can I get matches

Answer 1

While regular expressions can be good for a large variety of tasks, I find it usually falls short when parsing HTML DOM. The problem with HTML is that the structure of your document is so variable that it is hard to accurately (and by accurately I mean 100% success rate with no false positive) extract a tag.

What I recommend you do is use a DOM parser such as SimpleHTML and use it as such:

function get_first_image($html) {
    require_once('SimpleHTML.class.php')

    $post_html = str_get_html($html);

    $first_img = $post_html->find('img', 0);

    if($first_img !== null) {
        return $first_img->src;
    }

    return null;
}

Some may think this is overkill, but in the end, it will be easier to maintain and also allows for more extensibility. For example, using the DOM parser, I can also get the alt attribute.

A regular expression could be devised to achieve the same goal but would be limited in such way that it would force the alt attribute to be after the src or the opposite, and to overcome this limitation would add more complexity to the regular expression.

Also, consider the following. To properly match an tag using regular expressions and to get only the src attribute (captured in group 2), you need the following regular expression:

<\s*?img\s+[^>]*?\s*src\s*=\s*(["'])((\\?+.)*?)\1[^>]*?>

And then again, the above can fail if:

• The attribute or tag name is in capital and the i modifier is not used.
• Quotes are not used around the src attribute.
• Another attribute then src uses the > character somewhere in their value.
• Some other reason I have not foreseen.

So again, simply don't use regular expressions to parse a dom document.

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EDIT: If you want all the images:

function get_images($html){
    require_once('SimpleHTML.class.php')

    $post_dom = str_get_dom($html);

    $img_tags = $post_dom->find('img');

    $images = array();

    foreach($img_tags as $image) {
        $images[] = $image->src;
    }

    return $images;
}