Custom pagination for custom post types (by names)

Interesting question! I solved it by expanding the WHERE query with a bunch of post_title LIKE 'A%' OR post_title LIKE 'B%' ... clauses. You can also use a regular expression to do a range search, but I believe the database won’t be able to use an index then.

This is the core of the solution: a filter on the WHERE clause:

add_filter( 'posts_where', 'wpse18703_posts_where', 10, 2 );
function wpse18703_posts_where( $where, &$wp_query )
{
    if ( $letter_range = $wp_query->get( 'wpse18703_range' ) ) {
        global $wpdb;
        $letter_clauses = array();
        foreach ( $letter_range as $letter ) {
            $letter_clauses[] = $wpdb->posts. '.post_title LIKE \'' . $letter . '%\'';
        }
        $where .= ' AND (' . implode( ' OR ', $letter_clauses ) . ') ';
    }
    return $where;
}

Of course you don’t want to allow random external input in your query. That is why I have an input sanitization step on pre_get_posts, which converts two query variables into a valid range. (If you find a way to break this please leave a comment so I can correct it)

add_action( 'pre_get_posts', 'wpse18703_pre_get_posts' );
function wpse18703_pre_get_posts( &$wp_query )
{
    // Sanitize input
    $first_letter = $wp_query->get( 'wpse18725_first_letter' );
    $last_letter = $wp_query->get( 'wpse18725_last_letter' );
    if ( $first_letter || $last_letter ) {
        $first_letter = substr( strtoupper( $first_letter ), 0, 1 );
        $last_letter = substr( strtoupper( $last_letter ), 0, 1 );
        // Make sure the letters are valid
        // If only one letter is valid use only that letter, not a range
        if ( ! ( 'A' <= $first_letter && $first_letter <= 'Z' ) ) {
            $first_letter = $last_letter;
        }
        if ( ! ( 'A' <= $last_letter && $last_letter <= 'Z' ) ) {
            if ( $first_letter == $last_letter ) {
                // None of the letters are valid, don't do a range query
                return;
            }
            $last_letter = $first_letter;
        }
        $wp_query->set( 'posts_per_page', -1 );
        $wp_query->set( 'wpse18703_range', range( $first_letter, $last_letter ) );
    }
}

The final step is to create a pretty rewrite rule so you can go to example.com/posts/a-g/ or example.com/posts/a to see all posts beginning with this (range of) letter(s).

add_action( 'init', 'wpse18725_init' );
function wpse18725_init()
{
    add_rewrite_rule( 'posts/(\w)(-(\w))?/?', 'index.php?wpse18725_first_letter=$matches[1]&wpse18725_last_letter=$matches[3]', 'top' );
}

add_filter( 'query_vars', 'wpse18725_query_vars' );
function wpse18725_query_vars( $query_vars )
{
    $query_vars[] = 'wpse18725_first_letter';
    $query_vars[] = 'wpse18725_last_letter';
    return $query_vars;
}

You can change the rewrite rule pattern to start with something else. If this is for a custom post type, be sure to add &post_type=your_custom_post_type to the substitution (the second string, which starts with index.php).

Adding pagination links is left as an exercise for the reader 🙂