Anyone know mod_rewrite?

[kau]

I would like to use mod_rewrite, if possible, on a subdomain, to point to the member.php script according not to their userid but username.

So let's say the username attached to the userid of 1 is John.

I want;

subdomain.domain.com/John

I know I could do something like subdomain.domain.com/1John

So the script knows the userid but how would I do it with just passing the username?