Hi and welcome to LQ,
For this to work, you'll need:
Code:
AllowEncodedSlashes On
into httpd.conf or a vhost part.
Note that according to the documentation, it should work for both, but it didn't for me (apache 2.2.19). I had to put it into a <VirtualHost>...</VirtualHost> definition
Then you can use your rule (without the leading slash after ^):
Code:
RewriteRule ^(.*)REG_LOGINfromURL(.*) http://mydomain.com [L,R=301]
Regards