Using Standard REGEX to add a character at the end of a string

Question

I need to add a backward slash to a string before storing it;

e.g. source "user1@domain1" - i need to store this string into two variables, userid and domain. However for the domain variable, before storing it I want to add a backward slash to the end of the domain name e.g. "domain1\" how can this be done in regex, thanks.