[antlr-interest] Newbie Question about Syntactic Predicates

[hawkwall]

Hello,

I need to match the following data

NUMBER.OF.SURFACE: 3
NUMBER.OF.STANDALONE: 5

Where all I am really concered about is that the surface has a number
3, and the standalone has a number 5.  I put the following in my Lexer

DIGITS : (0..9)+ ;

DOT : '.' ;

COLON : ':' ;

SURFACE : "NUMBER" DOT "OF" DOT "SURFACE" COLON ;

STANDALONE : "NUMBER" DOT "OF" DOT "STANDALONE" COLON;

WS	:	(	' '
		|	'\t'
		|	'\f' 
		|	(	options {generateAmbigWarnings=false;}
			:	"\r\n"  // DOS
			|	'\r'    // Macintosh
			|	'\n'	// Unix
			)
			{newline();}
		)+

    // now the overall whitespace action -- skip it!
    { $setType(Token.SKIP); }
    ;

And my Parser looks like:

start :  rule1 rule2;

rule1 : SURFACE DIGITS ; 

rule2 : STANDALONE DIGITS ;

with some actions to print out the number it finds.  If k<12 in the
lexer, I get a nondeterminism error, and can see the problem in the
generated Lexer.  But k=12 takes awhile to generate the .java files. 
I can break this up into smaller tokens that are put together in the
parser like:

Lexer:
NUMBER : "NUMBER" ;
OF : "OF" ; 
DOT : '.' ;
STANDALONE : "STANDALONE" ;
SURFACE : "SURFACE" ;
etc.

and then in Parser:
start : rule1
        rule2

rule1 : NUMBER DOT OF DOT STANDALONE COLON DIGITS ; 
rule2 : NUMBER DOT OF DOT SURFACE COLON DIGITS ;

and it works with a smaller k value in the lexer, but it doesn't seem
like the best option and makes the parser harder to construct.  I have
read everything I can find about syntatic predicates, and they seem to
be what I need, but I can't get it work.  I added the following to the
Lexer from above

SURFACE_OR_STANDOFF
	: 	("NUMBER" DOT "OF" DOT "SURFACE"  ) => 
		"NUMBER" DOT "OF" DOT "SURFACE" DOT "TO" DOT "AIR" DOT "THREAT" DOT
"CLASSES" COLON 
		{$setType(SURFACE); }
	|	("NUMBER" DOT "OF" DOT "STANDOFF" DOT "RANGE" DOT "AIRCRAFT" DOT
"CLASSES" COLON ) => 
		"NUMBER" DOT "OF" DOT "STANDOFF"  
		{$setType(STANDOFF);}
	;

What am I missing?  Is there a better way to match large tokens?  I
can tell that the Parser is backtracking at all.  The error message I
get is:

parser exception: line 2:12: expecting 'U', found 'T'
line 2:12: expecting 'U', found 'T'

Which says to me the parser is still trying to match the SURFACE
token.  I tried to define the parser to match SURFACE once and then
STANDALONE, but my head hurts from all the banging.  Thanks for the
help.

Mike Wall


 

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