URL redirect to a different extension

RewriteEngine On

RewriteRule ^(.+).<#ext1#>$ /<#path/#>$1.<#ext2#> [NC,R=permanent,L]

URL redirect to a different page

RewriteEngine On

RewriteRule ^<#old-page.html#>$ /<#new-path/new-page.html#> [NC,R=permanent,L]

URL redirect to a different directory

RewriteEngine On

RewriteRule ^(.*)$ /<#new-path/#>$1 [R=permanent,L]

Although this is the simplest way to redirect all requests to a different directory, it blindly does the redirect to the new page. To verify that the target page exists, include a RewriteCond with -f:

RewriteEngine On

RewriteCond %{REQUEST_URI} ^/<#old-path/#>(.*)$ [NC]
RewriteCond %{DOCUMENT_ROOT}/<#new-path/#>%1 -f
RewriteRule ^(.*)$ /<#new-path/#>$1 [R=permanent,L]

RewriteCond %{DOCUMENT_ROOT}/<#new-path/#>index.html -f
RewriteRule ^(.*)$ /<#new-path/#> [R=permanent,L]

The RewriteCond with -f for the first RewriteRule will verify that the file for the target page exists before doing the redirection there. The %1 back reference in that condition matches the regular expression group ((.*)) in the RewriteCond above it. The RewriteCond for the second RewriteRule will verify that an index.html file exists before the following RewriteRule does a redirect of any request for a document that does not exist in the new location, including a directory level request for the DirectoryIndex document. That one can be omitted if you already know that the index.html file exists in the target directory.

URL redirect to a script

RewriteEngine On

RewriteCond %{REQUEST_URI} !\.<#ext#>
RewriteCond %{REQUEST_URI} ^/(.+)$
RewriteCond %{DOCUMENT_ROOT}/%1 !-d
RewriteCond %{DOCUMENT_ROOT}/%1 !-f
RewriteRule ^([^/]+)$ /index.<#ext#>?url=$1 [R=temp,L]

Here is a line-by-line explanation of that code:

1. Line 1 eliminates the index.ext file right off the bat, to avoid redirecting requests to the script itself.
2. In line 2, the %{REQUEST_URI}% will start with a "/" but it is not included in the subpattern, so the %1 back reference will not include the leading "/".
3. Line 3 makes sure a directory does not exist at the requested URL.
4. Line 4 makes sure a file does not exist at the requested URL.
5. Line 5 does a temporary redirect (HTTP 302) to the index.ext script with the original URL path as the url parameter in the query part of the rewritten URL.

HTTP Status Code Flags in Redirect RewriteRules

R

R=temp

R=302

Found - use See Other or Temporary Redirect at user agent's discretion

R=permanent

R=301

Moved Permanently (Permanent Redirect)

R=303

See Other

R=307

Temporary Redirect