un-en-dash.php

<?php
/*
Plugin Name: Un-en-dash the Double Dash
*/
add_filter( 'the_content' , 'mh_un_en_dash' , 50 );
add_filter( 'the_title', 'mh_un_en_dash' , 50 );
add_filter( 'the_content', 'mh_un_en_dash' , 50 );
add_filter( 'the_excerpt', 'mh_un_en_dash' , 50 );
add_filter( 'comment_text', 'mh_un_en_dash' , 50 );
add_filter( 'list_cats', 'mh_un_en_dash' , 50 );

function mh_un_en_dash( $text ) {
        $content = str_replace( '&#8211;' , '--' , $text );
        return $text;
}