厳密なE-mailアドレスチェックを書こうと思うと酷いことになる。

preg_match('/^(?:[^(\040)<>@,;:".\\\\\[\]\000-\037\x80-\xff]+(?![^(\040)<>@,;:"
.\\\\\[\]\000-\037\x80-\xff])|"[^\\\\\x80-\xff\n\015"]*(?:\\\\[^\x80-\xff]
[^\\\\\x80-\xff\n\015"]*)*")(?:\.(?:[^(\040)<>@,;:".\\\\\[\]\000-\037\x80-\xff]
+(?![^(\040)<>@,;:".\\\\\[\]\000-\037\x80-\xff])|"[^\\\\\x80-\xff\n\015"]*
(?:\\\\[^\x80-\xff][^\\\\\x80-\xff\n\015"]*)*"))*@(?:[^(\040)<>@,;:".\\\\\[\]
\000-\037\x80-\xff]+(?![^(\040)<>@,;:".\\\\\[\]\000-\037\x80-\xff])|
\[(?:[^\\\\\x80-\xff\n\015\[\]]|\\\\[^\x80-\xff])*\])(?:\.(?:[^(\040)<>@,;:"
.\\\\\[\]\000-\037\x80-\xff]+(?![^(\040)<>@,;:".\\\\\[\]\000-\037\x80-\xff])|
\[(?:[^\\\\\x80-\xff\n\015\[\]]|\\\\[^\x80-\xff])*\]))*$/', $email);