.htaccess and redirecting

I dont know if this is already posted on this forum before, but i saw it on another forum somewhere, but i didn't understand it verry wel. So if somebody could explain that would help alot.

Here is the situation:

if somebody goes to www.mydomain.com/name (for example www.mydomain.com/puttie) then the script has to redirect the user to www.mydomain.com/userpage.php?username=name (for example www.mydomain.com/userpage.php?username=puttie)

I think this has to be done with .htaccess, but i dont know how.

Thanks in advance

First off, you have got a real bad idea for redirecting www.mydomain.com/puttie to whatever simply because /puttie is the very top level relative URL. Everyone is unable to access your main site because a redirection will occur immediately. Instead, you should use a 2nd level relative URL like so:

www.mydomain.com/user/puttie to www.mydomain.com/userpage.php?username=puttie or
puttie.mydomain.com to www.mydomain.com/userpage.php?username=puttie (if wildcard is enabled in your DNS configuration)

Second, what about accessing to www.mydomain.com/user/puttie/dir/blahblah.html ? I don't think you really have any idea what you are trying to do.

Let's forget about your bad idea and use my 1st example:

#http://www.mydomain.com/.htaccess
#username must contain only a to z 0 to 9 chars
#when accessing to www.mydomain.com/user/username or www.mydomain.com/user/username/
RewriteEngine on
RewriteRule ^/user/([a-z0-9]+)([^/]*)$ userpage.php?username=$1 [R,L]

My 2nd example:
# Redirect http://puttie.mydomain.com or http://puttie.mydomain.com/ to http://www.mydomain.com/userpage.php?username=puttie
RewriteEngine on
RewriteCond %{HTTP_HOST} ^www\.mydomain\.com$ [NC]
RewriteRule ^(.+) - [L]
RewriteCond %{HTTP_HOST} ^[^.]+\mydomain\.com$ [NC]
RewriteRule ^(.+) %{HTTP_HOST}=$1 [C]
RewriteRule ^([^.]+)\.mydomain\.com(.*) http://www.mydomain.com/userpage.php?username=$1 [R,L]

>> I dont know if this is already posted on this forum before

Yes, dozen times already and you should do a search first.