wp_list_categories() by alphabet? (ex. only category titles that start with “A”) [closed]

wp_list_categories() uses the Walker_Category walker which applies the list_cats filter, so a little PHP and:

function alpha_prefix_wpse_210205($cat) {
  $fl = substr($cat,0,1);
  $fl = strtoupper($fl);
  return $fl.' '.$cat;
}
add_filter('list_cats','alpha_prefix_wpse_210205');

You will want to apply the filter just before you need it and remove it afterwards.

function show_categories_fn(){
    add_filter('list_cats','alpha_prefix_wpse_210205');
    $lc = wp_list_categories("echo=0&title_li");
    remove_filter('list_cats','alpha_prefix_wpse_210205');
    return $lc;
}
add_shortcode('show_categories', 'show_categories_fn');