You could do it with a regular expression: see Jeffrey Friedl's book for the example. The trouble with this is that RFC 822 allows a rather large number of different formats - the example mentioned above is 6598 bytes long (Appendix B).

I'd suggest that using a regex is a very bad idea, and I would use something along the lines of Email::Valid to do it for me. If you don't want to use modules (why?), then follow the advice revealed by typing

perldoc -q "How do I check a valid mail address?"
[download]

at your command prompt. (Adjust for your OS and perl installation.)
cheers
davis
Is this going out live?
No, Homer, very few cartoons are broadcast live - it's a terrible strain on the animator's wrist