Just like in Perl 5 you can use look-aheads:

token neither_a_nor_b {
   <?before <not_a>>
   <?before <not_b>>
   .
}
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If you define your define your character classes positively, you can also combine them directly:

token a { <[ a ]> }
token b { <[ b ]> }
token neither_a_nor_b { <-a-b> }
[download]

(don't know if anybody implements that, though.