Use custom tag for numeric entries

So I’m trying to figure out how to clump any terms that begin with a number into a custom tag like “Misc” so that I don’t wind up with 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, etc.

Looking in the “set_letter” method, that appears to be where you define the first letter of the word to be the tag the term is categorized under. However, when I tried to add a check for is_numeric, it doesn’t seem to work.

public function set_letter($post_ID, $post) {
		if ($post->post_type == 'dict-terms') {
			$lower = mb_strtolower($post->post_title);
			$one = mb_substr($lower, 0, 1);
			
      if (is_numeric($one)) {
        $set = wp_set_object_terms($post_id, 'Misc', 'dict-terms-letter');
        wp_update_term($set[0], 'dict-terms-letter', array(
          'name' => mb_strtoupper('Misc')
        ));
      }
      else {
  			$set = wp_set_object_terms($post_ID, $one, 'dict-terms-letter');
  			wp_update_term($set[0], 'dict-terms-letter', array(
  				'name' => mb_strtoupper($one),
  			));
      }
		}
		
		return;
	}