Don't. You will never end up with a valid expression.
For example these are all valid email addresses:
"Abc\@def"@example.com
"Fred Bloggs"@example.com
"Joe\\Blow"@example.com
"Abc@def"@example.com
customer/department=shipping@examp le.com
[email protected]
!def!xyz%[email protected]
[email protected]
matteo(this is a comment)[email protected]
root@[127.0.0.1]
Just to mention a few problems:
Before even beginning check the corresponding RFCs