[ 'SW', '1', 'A', ' ', '1', 'AA' ]

[ 'SW', '1', 'A' ]

[ 'SW', '22', ' ', '3' ]

$strs = ["SW1A 1AA", "SW1A", "SW22 3"];
foreach ($strs as $s) {
  if (preg_match_all('~[a-zA-Z]+|d+|[^da-zA-Z]+~', $s, $chunks)) {
    print_r($chunks[0]);
  }
}
// - Array ( [0] => SW [1] => 1 [2] => A [3] =>   [4] => 1 [5] => AA )
// - Array ( [0] => SW [1] => 1 [2] => A )
// - Array ( [0] => SW [1] => 22[2] =>   [3] => 3 )

[a-zA-Z]+|d+|[^da-zA-Z]+