I use these regular expressions all the time, but it’s much more convenient to have them both in convenient PHP functions.

// Returns only the file extension (without the period).
function file_ext($filename) {
	if( !preg_match('/\./', $filename) ) return '';
	return preg_replace('/^.*\./', '', $filename);
}

// Returns the file name, less the extension.
function file_ext_strip($filename){
    return preg_replace('/\.[^.]*$/', '', $filename);
}