At 09:27 17/02/2010, I wrote:
>It'd be nice if there was some way to express a negative match
>via a syntactic predicate, eg:
> FOOLIST: 'foo[' (('foo') => ~ | ID)+ ']';
>(where '~' in an alt basically means "break", ie. match nothing
>and terminate the innermost loop.)
>Or, perhaps better:
> FOOLIST: 'foo[' (('foo') ~=> ID)+ ']';
>(where '~=>' means "only take this path if the predicate
*fails*")
Another possible syntax might be to allow ~ to act on sequences as
well as sets. Then you could use either of these:
FOOLIST: 'foo[' ((~'foo') => ID)+ ']';
FOOLIST: 'foo[' ((~('f' 'o' 'o')) => ID)+ ']';
This approach might be even more useful than the first, although
it's harder to define what the result should be if you try to make
a sequence out of both negative and positive sequences (ie. what
should "~'foo' 'bar'" mean? Is it "any three characters except
'foo', followed by 'bar'", or "any number of characters except
'foo', followed by 'bar'"? Or just meaningless? What if it were
"~('foo'|'quux') 'bar'"? [Assuming this is all at lexer context,
so these are sequences of characters rather than individual
tokens.]).
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