The string should begin with "S2" followed by any 4 digits.

A matching string is "S20165".

I'm trying with the following code, but it always echos OK even when there are 5 or 6 digits.

```
$string='S20104';
if(preg_match('/S2[0-9]{4}/', $string)){
    echo 'OK';
}
else{
    echo 'NOT OK';
}

```