Sharepoint retrieving username without domain or programmtically removing domain

Assuming the user is returned in this format

domain\username

you can do the following:

string userWithoutDomain = userName.Substring(userName.IndexOf('\\') + 1);

If the format is like this

username@domain

then the following will work

string userWithoutDomain = user.Substring(0, user.IndexOf('@'));

Probably you should test which format you have and extract the user name based on that. If the user name contains neither a @ nor a \ then you just return the entire string.