How to list all record from one custom taxonomy start with only one letter i.e A

I want to list all records in one custom taxonomy start with only A or B. Below code is to list all record with all letters.

Here is the code to list all record with one custom taxonomy with all letters in groups.
Example,

<?php 
// Template Name: Store Template


// get all the stores
$stores = get_terms(APP_TAX_STORE, array('hide_empty' => 0, 'child_of' => 0, 'pad_counts' => 0, 'app_pad_counts' => 1));
// get ids of all hidden stores 
$hidden_stores = clpr_hidden_stores();
$list="";
$groups = array();


if ($stores && is_array($stores) ) {

    // unset child stores
    foreach($stores as $key => $value)
    if($value->parent != 0)
          unset($stores[$key]);

    foreach($stores as $store)
        $groups[mb_strtoupper(mb_substr($store->name, 0, 1))][] = $store;

    if (!empty($groups)) :

        foreach($groups as $letter => $stores) {
      $old_list = $list;
      $letter_items = false;
            $list .= "\n\t" . '<h2 class="stores">' . apply_filters( 'the_title', $letter ) . '</h2>';
            $list .= "\n\t" . '<ul class="stores">';

            foreach($stores as $store) {
                if (!in_array($store->term_id, $hidden_stores)) {
                    $list .= "\n\t\t" . '<li><a href="' . get_term_link($store, APP_TAX_STORE) . '">' . apply_filters('the_title', $store->name). '</a> (' . intval($store->count) . ')</li>';
          $letter_items = true;
        }
            }   

            $list .= "\n\t" . '</ul>';

      if(!$letter_items)
        $list = $old_list;
        }

    endif;

} else {

    $list .= "\n\t" . '<p>' . __('Sorry, but no stores were found.', 'appthemes') .'</p>';

}
?>

You can use preg_match function:

foreach($stores as $store)
    // if $store starts with A or B
    if( preg_match( '/^(A|B|a|b)/', $store ) )
        //I've not checked this one
        $groups[mb_strtoupper(mb_substr($store->name, 0, 1))][] = $store;