Negative Look-ahead problem

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reserved_word: 'AND' | 'OR' 
WORD:          ...!reserved_word /[a-z0-9]+/i 

Using the above grammar, rule WORD fails to match any token that start
with a reserved word (e.g. Andy). That is not what I expected since in
'demo_Cgrammar.pl', we have:

IDENTIFIER:     ...!reserved_word /[a-z]\w*/i

I am I missing something?

Thanks!
Andras