The RewriteRule Solution
To accomplish this expansion, you need to write a RewriteRule regular expression. First, you need to find the URI pattern /r/d, and then extract /d and turn it into /dhtml. Next, append a trailing slash.
Apache requires two directives to turn on and configure the mod_rewrite module: the RewriteEngine boolean and the RewriteRule directive. The RewriteRule is a regular expression that transforms one URI into another. The syntax is shown here:
RewriteRule <pattern> <rewrite as>
So to create a rewrite rule to solve this problem, you need to add the following two mod_rewrite directives to your server configuration file (httpd.conf or .htaccess):
RewriteEngine On RewriteRule ^/r/d(.*) /dhtml$1
This regular expression matches a URL that begins with /r/ (the ^ character at the beginning means to match from the beginning of the string). Following that pattern is d(.*), which matches one or more characters after the d.. Note that using /r/dodo would expand to /dhtmlodo, so you'll have to make sure anything after r/d always includes a /.
So when a request comes in for the URI <a href="/r/d/diner/">DHTML Diner</a>, this rule expands this abbreviated URI into <a href="/dhtml/diner/">DHTML Diner</a>.