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To add a maxlength attribute to the WordPress post title, you can use the maxlength parameter of the wp_editor() function.

In your theme’s functions.php file, add the following code:

function custom_title_maxlength() {
    add_filter( 'tiny_mce_before_init', 'custom_tiny_mce_before_init' );
}
add_action( 'admin_head-post.php', 'custom_title_maxlength' );
add_action( 'admin_head-post-new.php', 'custom_title_maxlength' );

function custom_tiny_mce_before_init( $init ) {
    $init['setup'] = "function (ed) {
        ed.on('init', function() {
            var wpTitle = document.getElementById('title');
            if(wpTitle) {
                wpTitle.setAttribute('maxlength', '100');
            }
        });
    }";
    return $init;
}

This code uses the tiny_mce_before_init filter to add a JavaScript function to the setup parameter of the TinyMCE editor initialization. The function then adds a maxlength attribute to the post title input element with an ID of title.

Change the number 100 to the desired character limit.