Matching any character except an underscore using Regex

I'm trying to write Regex that will match a string of characters EXCEPT strings with an underscore.

I have this so far /[A-Za-z0-9]+/ but I don't know what to include in it to make it require no underscore.

UPDATE:

Is should have made this more clear off the bat. I am trying to match an email address, but not email addresses that have an underscore in the portion after the _

This is what I have in total so far. /[A-Za-z_0-9]+@[A-Za-z0-9]+\.(com|ca|org|net)/ The answers as of yet, don't work