正则表达式：排除匹配

I have this pattern:

~^([a-z0-9]+[a-z0-9-]+[a-z0-9]+\.([a-z]+)(\.[a-z]+)?)$~i

It will match the following:

xxx.xxx or xxx.xxx.xxx

(number of x per doesn't matter)

How do I disallow xxx.xxx.xxx if the set of characters before the first . is wwww ?

For example, it should allow example.co or example.co.uk but it should not allow www.example.co or www.example.co.uk

You can use a negative lookahead/lookbehind, eg:

~^((?!www\.)[a-z0-9]+[a-z0-9-]+[a-z0-9]+\.([a-z]+)(\.[a-z]+)?)$~i