How do I redirect a request for a specific URL to the domain root using htaccess?

The short answer

RedirectMatch 301 ^/the-url-to-redirect/$ http://www.domain.tld/the-new-url/

The Problem

I have a WordPress blog in a subdirectory, but I want calls for the domain root to display content of the sub directory. However I also want the sub directory to display in the URL for all OTHER URL requests. Eg http://www.domain.tld/blog/page-url

Note:
WordPress has a built in mechanism to allow the blog to be installed in a sub directory but REMOVES THE SUB DIRECTORY FROM THE URL. See:
http://codex.wordpress.org/Giving_WordPress_Its_Own_Directory

The Solution

Follow the instruction in the above URL the above, COPY  (not move) to domain root the WordPress index.php file but change the line:

require(‘./wp-blog-header.php’);
to
require(‘./blog/wp-blog-header.php’);

But keep the  settings:

WordPress Address (URL) http://www.domain.tld/blog/
Site Address (URL) http://www.domain.tld/blog/

And leave the sub directory  htaccess file where it is

Problem 2

I now have a Duplicate Content issue with:
http://www.domain.tld and http://www.domain.tld/blog/

To solve this I can create a 301 permanent redirect for the sub directory (top of this page).

Problem 3

WordPress 2.9 onwards will automatically create canonical URLs in the header of every page. This is a useful addition, but in this case the page at http://www.domain.tld/blog/ carries the canonical tag for http://www.domain.tld/blog/ but that page now redirects to http://www.domain.tld/ creating a permananet loop between the canonocal link and the RedirectMatch 301. Is it a problem? Who knows but I would rather not risk losing my SEO juice over it by having a permanent loop

We can remove the automatically generated canonical links by placing the following in the themes functions.php file

# Remove WordPress canonical links
remove_action(‘wp_head’, ‘rel_canonical’);

But I don’t want to turn off ALL my canonical link tags, only the one on the sub directory, therefore I use this in the themes functions.php instead:

# Remove WordPress canonical from front_page
add_action(‘template_redirect’, ‘my_canonical’);
function my_canonical()
{
if(is_front_page())
{
remove_action( ‘wp_head’, ‘rel_canonical’ );
}
}

See: http://stackoverflow.com/questions/2202829/wordpress-init-remove-action-with-conditional-tags

Now we have:

http://www.domain.tld/ for the front page
http://www.domain.tld/blog/ no longer exists alone (it redirects to root)
http://www.domain.tld/blog/full-page-urls for all other pages

All pages carry:

<link rel=’canonical’ href=’http://www.domain.tld/blog/full-url-here’ />

Apart from: http://www.domain.tld/ which we can deal with separately for duplicate content issues.