$re = '~(?!<[^>]*?>)\bhello world\b(?![^<]*?(?:</[^/]*>|>))~';
$str = 'one two three hello world four five
one <a href=\'index.html\'> hello world </a> four five
foo bar <p>hello world</p> hello
<p name="hello world">hello world</p>
one two three <img alt=\'hello world\' \\>four five
<img alt="hello world"> foo
foo bar <a href="index.html">hello world</a> again
one two three hello world four five
one two three <img alt=\'hello world\' \\>four five';
preg_match_all($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