Bueno se me ocurre algo así, no sé si esté dando mucha vuelta pero funciona:
Cita: /^(?=^\w+[.\-+_]{1}\w+$)[\w.+_\-]{3,63}$/
Negrita es lo capturado:
Cita: asss+aaas
aas++fdfs
aaaa..aaaa
aaa.aaa
aaaaaaa
20.20
df+df3ffff%dsfsdfsfsfdf@
[email protected] test_test test-test
fubar
[email protected]
aaa@aaa
aaaaaa.aaaaaaaa
.aaaaa
a.a a.aaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaa.a aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaa aaaaaaaaaaaaa (63 caracteres, ignorar espacios que salen por el foro¿?)
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.aaaaaaaaaaaaaaaa aaaaaaaaaaaaaa
(64 caracteres)
aa.aa.aa.aaaaa.aaaa.aaaa.aaaa
PD: No está probada al 100%
Saludos.