Improve constraints to add matching

Need docs
How about moving this and also the `regexp.MatchString` below into its own `match()` function?

It should take care of doing regexp matching + ToLower() etc so you could simply write:
`if f.match(v, node.ID) || f.match(v, node.Name) {`

Eventually, the function could take care of handling the operators as well (`==`, `!=`).