Below is good tested regex for validating domain name

(([a-zA-Z0-9]|%[a-fA-F0-9]{2})(-([a-zA-Z0-9]|%[a-fA-F0-9]{2})+)?\.)+([a-zA-Z]{2,7}\.?)

The following regex is very simple and i have used it for validating domain names

/^([a-z0-9][a-z0-9\-]*[a-z0-9]\.){2,}$/

Thought of taking the discussion further you can try this regex

/^([a-z0-9]([-a-z0-9]*[a-z0-9])?\.)+((a[cdefgilmnoqrstuwxz]|aero|arpa)|
(b[abdefghijmnorstvwyz]|biz)|(c[acdfghiklmnorsuvxyz]|cat|com|coop)|
d[ejkmoz]|(e[ceghrstu]|edu)|f[ijkmor]|(g[abdefghilmnpqrstuwy]|gov)|
h[kmnrtu]|(i[delmnoqrst]|info|int)|(j[emop]|jobs)|k[eghimnprwyz]|
l[abcikrstuvy]|(m[acdghklmnopqrstuvwxyz]|mil|mobi|museum)|
(n[acefgilopruz]|name|net)|(om|org)|(p[aefghklmnrstwy]|pro)|
qa|r[eouw]|s[abcdeghijklmnortvyz]|(t[cdfghjklmnoprtvwz]|travel)|
u[agkmsyz]|v[aceginu]|w[fs]|y[etu]|z[amw])$/i