Bernardo,
Definitely let us know if the above fixes it or not. We can definitely help you further however you need.
Also, if you don't mind, how are you doing the regex? Just curious as to what plugin your using, or if it's something custom. We had a question (that we helped a user solve) when using the Registration Validation plugin, and we can help in that area if you're using the same plugin.
Thanks, and good luck!