[antlr-interest] Lexer non-determinism
Terence Parr
parrt at jguru.com
Sun Dec 22 12:56:43 PST 2002
On Sunday, December 22, 2002, at 08:19 AM, seaswar <sje at sigma-inc.com>
wrote:
> I posted this on the jGuru forum but got no reply. Perhaps I'll have
> better luck here.
>
> I don't understand why the following is ambiguous for k=2 but notfor
> k=3. I checked the generated code (java, antlr 2.7.1) and it looks
> right. So I turned of the warning. But I would still like to
> understand why there is a warning in the first place. No warning is
> generated for k=3.
>
> class TestLexer extends Lexer;
>
> options { k = 2; }
>
> MULTI_LINE_COMMENT
> : "/*"
> ( { LA(2) != '/' }? '*'
> | ( "\r\n" | '\n') { newline(); }
> | ~( '*' | '\r' | '\n')
> )*
> "*/" { $setType(Token.SKIP); }
> ;
>
> ANTLR Parser Generator Version 2.7.1 1989-2000 jGuru.com
> com/qsi/parser/test/test9.g:7: warning: lexical nondeterminism upon
> com/qsi/parser/test/test9.g:7: k==1:'*'
> com/qsi/parser/test/test9.g:7: k==2:'/'
> com/qsi/parser/test/test9.g:7: between alt 1 and exit branch of block
The predicate should hush the warning I think; perhaps 2.7.2 does.
Anyway, check the java grammar to see what it does.
Thanks,
Terence
--
Co-founder, http://www.jguru.com
Creator, ANTLR Parser Generator: http://www.antlr.org
Lecturer in Comp. Sci., University of San Francisco
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