Shortcode – Regex – SQL problem

I am creating a shortcode that displays a list of custom post type based on letter parameter. like [affiliate-mall letter=’a’] will display all letter A.

Problem is , how can I set the default/no parameter ex: [affiliate-mall], to query any entries having title that starts with a number or any symbol. I know this is done using regex but I don’t have an idea how to implement this. any hints?THanks! Code below

function custom_shortcode( $atts ) {

          // Attributes
          extract( shortcode_atts(
            array(
              'letter' => ''
            ), $atts )

      );

    $finalArgs =  array (
            'posts_per_page'=> 200,
            'order' => 'ASC',
            'post_type' => 'Affiliate'
        );
       global $wpdb;
        // Create a new instance
        $searchAffiliate = new WP_Query( $finalArgs );

        $mypostids = $wpdb->get_col("select ID from $wpdb->posts where post_title LIKE '".$letter."%' ");

        $args = array(
            'post__in'=> $mypostids,
            'post_type'=>'Affiliate',
            'orderby'=>'title',
            'order'=>'asc'
        );

        $res = new WP_Query($args);
         $count = 0;

        while( $res->have_posts() ) : $res->the_post();

            global $post;
            $count++;
           $client_logo = get_post_meta( $post->ID, '_cmb_affiliate_logo_image', true );
                   $affiliate_link = esc_url(get_post_meta( $post->ID, '_cmb_affiliate_link', true ));
                   if($count % 4 === 0){
                         $output .=  '<div class="one-fourth" style="padding-left:0;"><a href="' . $affiliate_link . '" target="_blank"><img src="' . $client_logo . '" /></a></div>'; 

                   }else{
                        $output .=  '<div class="one-fourth"><a href="' . $affiliate_link . '" target="_blank"><img src="' . $client_logo . '" /></a></div>'; 

                   }

        endwhile;
         if(!$output){
          $finalOutput = "No Entries for Letter $letter found";
        }else{
          $finalOutput = '<div style="clear:both;float:left;width:100%;">' .  $output . '</div>';
        }

         wp_reset_query();
         return $finalOutput;
    }
    add_shortcode( 'affiliate-mall', 'custom_shortcode' );
    ?>