ASN.1/SMI comment syntax on ANTLR 3 - antlr

Question

On ANTLR 2, the comment syntax is like this,
// Single-line comments
SL_COMMENT
: (options {warnWhenFollowAmbig=false;}
: '--'( { LA(2)!='-' }? '-' | ~('-'|'\n'|'\r'))* ( (('\r')? '\n') { newline(); }| '--') )
{$setType(Token.SKIP); }
;
However, when porting this to ANTLR 3,
SL_COMMENT
: (
: '--'( { input.LA(2)!='-' }? '-' | ~('-'|'\n'|'\r'))* ( (('\r')? '\n') | '--') )
{$channel = HIDDEN;}
;
because there is no more options {warnWhenFollowAmbig=false;}, the following comment cannot be parsed correctly,
-- some comment -- some not comment
Then, what is the possible way to define this SL_COMMENT rule for ANTLR 3?

Answer 1

Personally, I like to keep grammar rules as "empty" as possible. In this case, I would create a lexer method that returns true if the next two characters in the input are "--". As long as this is not the case, match any character other than \r and \n, and repeat that zero or more times until an optional "--" is encountered. Note that I didn't put a new line at the end because there is not necessarily a new line at the end (it could also be a EOF). Besides, \r and \n will likely be matched by a SPACE rule which is put on the HIDDEN channel: so there's no harm in doing it as I suggest.
A demo:
...
#lexer::members {
private boolean endCommentAhead() {
return input.LA(1) == '-' && input.LA(2) == '-';
}
}
...
SL_COMMENT
: '--' ({!endCommentAhead()}?=> ~('\r' | '\n'))* '--'?
;
...
And if you don't like the lexer members-block, you simply do:
SL_COMMENT
: '--' ({!(input.LA(1) == '-' && input.LA(2) == '-')}?=> ~('\r' | '\n'))* '--'?
;
EDIT
A small, complete demo:
grammar T;
#parser::members {
public static void main(String[] args) throws Exception {
String source = "12 - 34 -- foo - bar -- 42 \n - - 5678 -- more comments 666\n--\n--";
TLexer lexer = new TLexer(new ANTLRStringStream(source));
TParser parser = new TParser(new CommonTokenStream(lexer));
parser.parse();
}
}
#lexer::members {
private boolean endCommentAhead() {
return input.LA(1) == '-' && input.LA(2) == '-';
}
}
parse
: (t=. {System.out.printf("\%-15s\%s\n", tokenNames[$t.type], $t.text);})* EOF
;
SL_COMMENT
: '--' ({!endCommentAhead()}?=> ~('\r' | '\n'))* '--'?
;
MINUS
: '-'
;
INT
: '0'..'9'+
;
SPACE
: (' ' | '\t' | '\r' | '\n') {skip();}
;
which, after parsing the input:
12 - 34 -- foo - bar -- 42
- - 5678 -- more comments 666
will print:
INT 12
MINUS -
INT 34
SL_COMMENT -- foo - bar --
INT 42
MINUS -
MINUS -
INT 5678
SL_COMMENT -- more comments 666
SL_COMMENT --
SL_COMMENT --

Answer 2

I came across a solution finally,
SL_COMMENT
: COMMENT ( ({input.LA(2) != '-'}? '-') => '-' | ~('-'|'\n'|'\r'))* ( (('\r')? '\n') | COMMENT)
{ $channel = HIDDEN; }
;