How can I negate this express : *I have absolutely no clue how to work with regex's. I am less than a beginner.* 

I want to find any invalid css names from a string, so I can exclude them. Looking online I found a way to select the valid names. 

What I can't do is negate this expression: https://regexr.com/3ik89

Seems like everything I try just breaks it. 