Raku regex assert doesn't match

Hello, I have a Raku regex question.

See the following:

     token nonquoted_alphanumeric_text
     {
         <!before [null | false | true] <wb>>
         <[ A..Z _ a..z ]> <[ 0..9 A..Z _ a..z ]>*
     }

What I want is for "nonquoted_alphanumeric_text" to match any simple ASCII 
bareword EXCEPT a few special cases indicated in the example.

I was hoping there might be some better way of specifying this than my example.

Is there any more direct way in Raku to say, match this pattern initially, but 
if the result equals these exceptional values then treat it as having not matched.

I am looking for a fully declarative solution in the grammar itself, not 
something involving post-processing.

Thank you.

-- Darren Duncan