Well, I'm merely a demi-god of Parse::RecDescent {grin}, but I'd say a negative lookahead is what you need.

XXX: ...!('a' | 'this string' | ZZZ) YYY | 'something' 'else'
YYY: 'some string'
ZZZ: 'ww' | 'xx' | 'yy' | 'zz'
[download]

Beware that this is probably pretty slow.

-- Randal L. Schwartz, Perl hacker