Far from pretending this to be the ideal regex to match urls, the particular problem of only matching urls which do not begin with www can be resolved with a negative lookahead : (?!www\.)
meaning that the following text should not be www.
Therefore, your regex adapted with this negative lookahead:
^(?!www\.)[A-Za-z0-9_-]+\.+[A-Za-z0-9.\/%&=\?_:;-]+$
matches the following urls
google.com
google.com/
goggle.com/q
google.com/q?q=1&h=en
google.co.uk
google.co.uk/
goggle.co.uk/q
google.co.uk/q?q=1&h=en
w.google.com
w.google.com/
w.goggle.com/q
w.google.com/q?q=1&h=en
209.85.149.105
209.85.149.105/
209.85.149.105/q
209.85.149.105/q?q=1&h=en
209.85.149.105:80
209.85.149.105:80/
209.85.149.105:80/q
209.85.149.105:80/q?q=1&h=en
example.domain.bogus.number.of.subdomains/index.htm
but not those:
www.google.com
www.google.com/
www.google.co.uk
www.google.co.uk/
www.google.co.uk/q
www.google.co.uk/q?q=1&h=en
http://209.85.149.105/
http://209.85.149.105:80/
http://www.google.com
https://www.google.com
http://google.com
https://google.com