replace html entities in posts between pre tags

This one should work

add_filter( 'the_content', 'pre_content_filter', 0 );

function pre_content_filter( $content ) {
    return preg_replace_callback( '|<pre.*>(.*)</pre|isU' , 'convert_pre_entities', $content );
}

function convert_pre_entities( $matches ) {
    return str_replace( $matches[1], html_entity_decode( $matches[1] ), $matches[0] );
}

Let me know