I've been asked to develop a mod_rewrite to change
http://www.example.com/health-and-fitness-tips/999/
into
http://www.example.com/health-and-fitness-tips/how-do-I-lose-10kg-in-12-weeks/
where 999 is the id and How do I lose 10kg in 12 weeks is the title.
Is this even remotely possible or is mod_rewrite only for changing something like
http://localhost/siteengine/products.php?id=123
into
http://localhost/products/123
???
That's not practically do-able AFAIK ( dynamic-wise, without extra modules ). The first step you need to do is convert all the links to SEO ones, then you need to feed the titles as a parameter to a page.
Example:
<a href=/health-and-fitness-tips/how-do-I-lose/>
Condition: ^/health-and-fitness-tips/(.*)$/
Rewrite to: ^/page.php?title=$1
Some of this is pseudo code, but parse $_GET['title'] in page.php and query the database based on the permalink.
You'll also need to setup 301s for each permalinked URI so search engines don't get lost after you've done this.
Mod_rewrite just applies regular expression to the URL so getting ID from Title or vai versus is not possible (as far as I know).
You could do this, but it would be messy and laggy. You'd still need a script to perform the rewriting.
Check out the documentation for the RewriteMap directive, in particular the bit on the prg map type -- using an external program to provide URL mapping services.
Now, mind you, I think this is a bad, bad idea, but it would let you get this done.
i'm guessing the answer meder gave is the one you're looking for but technically you can create a static map file to redirect a set of title strings to ids and it doesn't have to execute an external prg executable:
RewriteMap static-title-to-id txt:/tmp/title_to_id.txt
RewriteRule ^/health-and-fitness-tips/(.*)/ /health-and-fitness-tips/${static-title-to-id:$1}/ [L]
with the contents of the /tmp/title_to_id.txt file something like this:
how-do-I-lose-10kg-in-12-weeks 999
some-other-title 988
and-another 983