Post titles not letting me use 2 consecutive dashes in permalink

Regarding the why part, I think I’ve traced this to the following line:

$title = preg_replace('|-+|', '-', $title);

within the sanitize_title_with_dashes() function.

It’s added to the santize_title() via the filter:

add_filter( 'sanitize_title', 'sanitize_title_with_dashes', 10, 3 );

but I wouldn’t recommend removing it totally.

Update:

Here’s a quick and dirty way around this:

add_filter( 'sanitize_title', function( $title )
{
    return str_replace( '--', 'twotempdashes', $title );
},9 );


add_filter( 'sanitize_title', function( $title )
{
    return str_replace( 'twotempdashes', '--', $title );
},11 );

where we replace two dashes into some temporary string, before the sanitize_title_with_dashes is activated and then replace the string again to two dashes afterwards. We could adjust this to the more general case, but you get the idea 😉