Finding and replacing strings with certain character patterns in array

I have a pretty large database with some data listed in this format, mixed up with another bunch of words in the keywords column.

BA 093, RJ 342, ES 324, etc.

The characters themselves always vary but the structure remains the same. I would like to change all of the strings that obey this character structure : 2 characters A-Z, space, 3 characters 0-9 to the following:

BA-093, RJ-342, ES-324, etc.

Be mindful that these strings are mixed up with a bunch of other strings, so I need to isolate them before replacing the empty space.

I have written the beginning of the script that picks up all the data and shows it on the browser to find a solution, but I'm unsure on what to do with my if statement to isolate the strings and replace them.

<?php

require 'includes/connect.php';

$pullkeywords = $db->query("SELECT keywords FROM main");

while ($result = $pullkeywords->fetch_object()) {

    $separatekeywords = explode(" ", $result->keywords);
    print_r ($separatekeywords);
    echo "<br />";
}

Any help is appreciated. Thank you in advance.

via Chebli Mohamed