$re = '/(<script.*src=)("|\')(.*)(\'|")/';
$str = '<script type="javascript" src=\'oldscript.js\' ></script>
<img src=\'test.jpg\' />';
$subst = "\1\2myscript.js\4";
$result = preg_replace($re, $subst, $str, 1);
echo "The result of the substitution is ".$result;
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