Need help rewriting URL's (Using htaccess)

Hi,

I'm trying to rewrite dynamic URL's to virtual .html URLs but can' figure it out..

example:

http:///www.example.com/sub/index.html?keyword=var1 var2 var3

Code (markup):

to

http:///www.example.com/sub/var1_var2_var3.html

Code (markup):

There could be more than 3 variables or less...

any idea?