$re = '/^f( (\d+)(\/{0,2})(\d*)(\/{0,2})(\d*)(\/{0,2})){0,3}$/m';
$str = 'f 1 2 3
f 1/ 2/ 3/
f 1// 2// 3//
f 1/1 2/2 3/3
f 1/1/ 2/2/ 3/3/
f 1//1 2//2 3//3
f /1/ /2/ /3/
f 1/1/1 2/2/2 3//3
f 1/1 2/2 3/3 4/4
^f(([ ]\\d{1,2}(\\/{0,2}\\d{0,1})\\/{0,1}){0,3})$
';
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