Redirecting Subdirectory to Subdomain with htaccess

A lot of web hosts today provide an easy implemention for subdomain creation in their administration panels. You just need to to go there, choose you subdomain name, and then point it to a directory in your tree.

If you can't, then it will be a little more complicated (You will need to resolve that subdomain to your server ip, configure some virtual hosts ... etc) and you may not have enough privileges to do that (unless you are on a dedicated server).

Edit 2

To redirect requests to www.example.com/blog to blog.example.com, try this :

RewriteEngine on RewriteCond %{HTTP_HOST} ^(www\.)?example\.com$ RewriteRule ^blog/(.*)$ http://blog.example.com/$1 [L,QSA,R=301] RewriteCond %{HTTP_HOST} ^blog\.example\.com$ RewriteCond %{REQUEST_URI} !^blog/ RewriteRule ^(.*)$ /blog/$1 [L,QSA] 

Have you tried this one?

RewriteEngine on RewriteBase / RewriteRule ^/blog/(.*)$ http://blog.subdomain.com/$1 [R=301,L] 

I wanted to add my two cents,

1) to answer the question above, this rewrite should fix it:

RewriteEngine on RewriteCond %{REQUEST_URI} !\. RewriteRule ^/blog$ http://blog.example.com [R=302,L] 

2) but, I think this is not enough by itself, you also need to change DNS, so that blog.example.com is pointed at the right server, and this can be done by a cname similar to this:

blog.example.com CNAME example.com TTL 1080 

(not exactly how it will look, but use your DNS webinterface to set this up).

RewriteCond %{HTTP_HOST} ^check.domain.info$

RewriteCond %{REQUEST_URI} !^/check/

RewriteRule (.*) /check/$1

To redirect subdomain1 and subdomain2 and directory3 to a directory with HTTPS://, I use the following code:

RewriteEngine on RewriteCond %{HTTP_HOST} ^subdomain1.example.com [OR] RewriteCond %{HTTP_HOST} ^subdomain2.example.com [OR] RewriteCond %{HTTP_HOST} ^example\.com/subdomain3 [NC] RewriteRule ^(.*)$ https://example.com/subdirectory/$1 [R=301,L]