$re = '/(?:href=|src=|url\()(?:[\",\']?)(.*image3\.png)[\",\',\)]+?/';
$str = '<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<title></title>
<meta name="description" content="" />
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1" />
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<link rel="stylesheet" type="text/css" href="style.css" />
</head>
<body>
<div class="wrapper">
<img src="image1.png" alt="description goes here">
<img src="image2.png" alt="description goes here">
<img src="image3.png" alt="description goes here">
</div>
<p>This "image3.png" is not linked so it should be ignored</>
<p>This "app.js" is not linked so it should be ignored</>
<p>This "style.css" is not linked so it should be ignored</>
<script src="app.js"></script>
</body>
</html>
';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php