via. https://gatespace.jp/2013/07/17/neta-wordpress-plugin/

content_mayuge_systems.php

<?php
/**
 * @package content_mayuge_systems
 * @version 0.1
 */
/*
Plugin Name: Content Mayuge Systems
Plugin URI: https://gist.github.com/gatespace/6007495
Description: 本文中に句点「。」「まゆげ」「マユゲ」「まゆ毛」「眉毛」があると「( ･ิω･)╯ิ」がくっつきます。
Author: gatespace
Version: 0.1
Author URI: http://gatespace.jp/
*/

add_filter('the_content', 'content_mayuge_systems');
function content_mayuge_systems( $content ) {
	$pattern = '/(。|まゆげ|マユゲ|まゆ毛|眉毛)/';
	$replace = '$1 ( ･ิω･)╯ิ ';
	$content = preg_replace( $pattern , $replace , $content );
	return $content;
}