Is there an easy way to convert jQuery code to regular javascript? I guess without having to access or understand the jQuery source code.
The easiest way is to just learn how to do DOM traversing and manipulation with the plain DOM api (you would probably call this: normal JavaScript).
This can however be a pain for some things. (which is why libraries were invented in the first place).
Googling for "javascript DOM traversing/manipulation" should present you with plenty of helpful (and some less helpful) resources.
The articles on this website are pretty good: http://www.htmlgoodies.com/primers/jsp/
And as Nosredna points out in the comments: be sure to test in all browsers, because now jQuery won't be handling the inconsistencies for you.