Why is my PHP not validating?

Question

I am trying to make it so that if a person inputs the wrong postal code format or leaves the input null it will give an error. I am trying to figure out why it isn't working. Please help!

<td>Postal Code</td>
<td>
    <input type="text" id="postal_code" name="postal_code" value="<?php if($_POST['postal_code'] == null){  echo '';}else{echo $_POST['postal_code'];}?>"/>
</td>
<td class="error" colspan="2">
    <?php 
        if(isset($_POST['send_bulkform'])  && $_POST['postalcode'] != preg_match("[A\-Za\-z][0\-9][A\-Za\-z] [0\-9][A\-Za\-z][0\-9]",$_POST['postal_code']) || isset($_POST['send_studentform']) && $_POST['postalcode'] != preg_match("[A\-Za\-z][0\-9][A\-Za\-z] [0\-9][A\-Za\-z][0\-9]",$_POST['postal_code']))
        { 
            echo "Required, ex. A1A 1A1."; 
        }
        elseif(isset($_POST['send_bulkform'])  && $_POST['postalcode'] == null || isset($_POST['send_studentform']) && $_POST['postalcode'] == null)
        {
            echo "Required, ex. A1A 1A1."; 
        }
    ?>
</td>
</tr>

Answer 1

Your regex is incorrect, and your PHP usage is incorrect. In PHP regexs need delimiters.

A backslash in a PCRE regex escapes the next character. I would use:

[A-Z]\d[A-Z] \d[A-Z]\d

and in PHP (add anchors so the whole string has to match):

preg_match('/^[A-Z]\d[A-Z] \d[A-Z]\d$/', $string);

If the case is optional add either the i modifier after the delimiter:

preg_match('/^[A-Z]\d[A-Z] \d[A-Z]\d$/i', $string);

or add the lowercase character range as well:

preg_match('/^[A-Za-z]\d[A-Za-z] \d[A-Za-z]\d$/', $string);

Regex Demo: https://regex101.com/r/cG9sG9/1
PHP Demo: https://eval.in/594444 (the 1 is true, for a match)

If the whitespace between the first grouping and second can vary use \h and either the * quantifier or the + quantifier. The * makes it not required (zero or more occurrences).