Simple Metabox Example w/ 20 Character Validation Error

js_metabox_example.php

<?php
/**
 * Simple Metabox Example
 *
 * Validates string is under 20 characters
 * 
 * Copyright (C) 2013 Jeff Sebring <http://jeffsebring.com>
 *
 *	This program is free software: you can redistribute it and/or modify
 *	it under the terms of the GNU General Public License as published by
 *	the Free Software Foundation, either version 3 of the License, or
 *	(at your option) any later version.
 *
 *	This program is distributed in the hope that it will be useful,
 *	but WITHOUT ANY WARRANTY; without even the implied warranty of
 *	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *	GNU General Public License for more details.
 *
 *	You should have received a copy of the GNU General Public License
 *	along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

// Safety First
defined( 'ABSPATH' ) || die;

// Add Metabox
add_action( 'add_meta_boxes', 'js_add_metabox_example' );
function js_add_metabox_example()
{

	global $post;

	$front_page = get_option( 'page_on_front' );

	if ( $post->ID === $front_page )
		add_meta_box( 'js_metabox_example', 'Metabox Example', 'js_metabox_example_box', 'page', 'normal', 'high' );

}

// Meta Box Input
function js_metabox_example_box( $post )
{

	$saved = get_post_meta( $post->ID, 'js_metabox_example', true );

	echo '<input type="hidden" name="js_metabox_example_nonce" value="', wp_create_nonce( basename( __FILE__ ) ), '" />';
	echo '<label for="js_metabox_example">Meta Data<label>';
	echo '<input type="text" id="js_metabox_example" name="js_metabox_example" value="' . $saved . '" size="25" />';

}

// Save Metabox Data
add_action( 'save_post', 'js_metabox_example_save' );
function js_metabox_example_save( $post_id )
{

	// Check User Capability
	if ( ! current_user_can( 'edit_page', $post_id ) )
	       return;

	// Check Nonce
	if ( ! wp_verify_nonce( $_POST[ 'js_metabox_example_nonce' ], basename( __FILE__ ) ) )
		return;

	// Check Autosave
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
		return;

	if ( isset( $_POST[ 'js_metabox_example' ] ) && strlen( $_POST[ 'js_metabox_example' ] ) > 20 ) :

		update_option( 'js_metabox_example_error', true );
		return;

	else :
		
		update_option( 'js_metabox_example_error', false );

	endif;

	update_post_meta(
		$_POST['post_ID' ],
		'js_metabox_example',
		sanitize_text_field( $_POST['js_metabox_example'] )
	);

}

// Check for Failed Validation
if ( get_option( 'js_metabox_example_error' ) )
	add_action( 'admin_notices', 'js_metabox_example_error' );

// Validation Error Message
function js_metabox_example_error()
{

	echo '<div id="message" class="error below-h2"><p>Meta Data must be at least 20 characters</p></div>';

}