PREG_MATCH and server-side script

Question

I would like to ask a question about preg_match and how the information about the "date" is actually determined to be in compliance with the regular expression, or not in compliance with the regex. Even after months of doing PHP and regex, I am just not clear on this issue. My question is, are the two regular expressions below equivalent? Notice that the first is a PREG_MATCH, while the second is a NOT PREG_MATCH. When a user mistakenly enters a date like 10-10-2014 which is in the wrong format, is the data actually sent to the server and THEN rejected by the server as being non-compliant with the regex, OR is the data stopped from being sent to the server in the first place? What I'm trying to understand is, does $date = $_POST['date'] refer to the act of the browser SENDING user data TO the server, OR does it refer to the act of the browser FETCHING user data FROM the server? In the first alternative below, does the server even "see" the wrongly-formatted date, or does the browser STOP the wrongly-formatted date from being sent to the server in the first place? Are the two things below exactly equivalent?

Alternative number 1:

   if (preg_match("/^(19|20)\d\d[- /.](0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])$/, "$date"){
    $date = $_POST['date'];
    }

Alternative number 2:

    if (!preg_match("/^(19|20)\d\d[- /.](0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])$/, "$date"){
    $date = "";
    $dateErr = "Invalid email format";
    }

Answer 1

PHP is server side scripting language, so data is sent to server and then manipulated (checked). For browser based validation You need to use JavaScript.

Answer 2

PHP is a server-side language. So, if PHP is processing data, that data has -- by definition -- already reached the server. The code above does not prevent anything from being sent to the server; it merely rejects it after arrival.

I'm unclear what you're asking about the regexes. They are literally identical; only the if statements are different. The if statements provided don't prevent the server from receiving data; they just determine what to do with it after arrival.