Negative regex validation operator

[amir-zeldes]

It should be possible to define negative regex matching for validation rules, e.g. to prevent hi_rend="italics" (supposing we want it to read "italic" if that value is used), we could do this:

, , ether, hi_rend, !~, italics

Where the operator is !~, and 'italics' is a regex pattern. The same operator should intuitively work for metadata validation too.