Function to check if a string is a date

If that's your whole string, then just try parsing it:

if (DateTime::createFromFormat('Y-m-d H:i:s', $myString) !== false) {
  // it's a date
}

Easiest way to check if a string is a date:

if(strtotime($date_string)){
    // it's in date format
}

Here's a different approach without using a regex:

function check_your_datetime($x) {
    return (date('Y-m-d H:i:s', strtotime($x)) == $x);
}