Help with regex

I need help with a regex that will match the following below where the only thing constant is 789- at the beginning and @domain.org at the end and the = sign in the middle. Any ideas?

Thanks

789-1-John.Smith=domain1.eu@domain.org
789-345-Jane.Clark=domain2.net@domain.org
789-9000-Mike.Smith=domain3.tv@domain.org