Match string that doesn't contain a specific word

I'm working with ruby with the match method and I want to match an URL that doesn't contain a certain string with a regular Expression: ex:

http://website1.com/url_with_some_words.html
http://website2.com/url_with_some_other_words.html
http://website3.com/url_with_the_word_dog.html

I want to match the URLs that doesn't contain the word dog, so the 1st and the 2nd ones should be matched