I have found this to be the best solution:
/^[^\s@]+@[^\s@]+\.[^\s@]+$/
It allows the following formats:
1. [email protected] 2. [email protected] 3. [email protected] 4. [email protected] 9. #!$%&'*+-/=?^_`{}|[email protected] 6. "()[]:,;@\\\"!#$%&'*+-/=?^_`{}| ~.a"@example.org 7. " "@example.org (space between the quotes) 8. üñîçøðé@example.com (Unicode characters in local part) 9. üñîçøðé@üñîçøðé.com (Unicode characters in domain part) 10. Pelé@example.com (Latin) 11. δοκιμή@παράδειγμα.δοκιμή (Greek) 12. 我買@屋企.香港 (Chinese) 13. 甲斐@黒川.日本 (Japanese) 14. чебурашка@ящик-с-апельсинами.рф (Cyrillic)
It's clearly versatile and allows the all-important international characters, while still enforcing the basic [email protected] format. It will block spaces which are technically allowed by RFC, but they are so rare that I'm happy to do this.