$re = '/\b0(px|r?em)/';
$str = '.foo {
padding: 0px;
}
.bar {
padding: 10px;
}
.baz {
margin-bottom: 0rem;
padding-left: 123px;
}';
preg_match_all($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