[antlr-interest] NoViableAltException in Grammar

[Bhisham Dutt]

Hi,
  I have the following grammar,  If i pass the following expression everything looks ok -- formula=-0.79*VAR+45.0  
But if  formula=-0.79*VAR-45.0   (With Negative operator is passed, I am getting NoViableAltException)
 
 
grammar Sample1;

options 
{
language = Java;
output = AST;
}

rule
: 'formula' '=' ((DOUBLE | NEGDOUBLE) '*')? 'VAR' (('-' | '+') DOUBLE)? 
;

fragment DIGIT : '0'..'9';
DOUBLE : (DIGIT)* ('.' (DIGIT)*)?
;

NEGDOUBLE : '-'(DIGIT)* ('.' (DIGIT)*)?
;

WS : (' ' | '\t' | '\n' | '\r' | '\f')+ {$channel = HIDDEN;};
COMMENT : '//' .* ('\n'|'\r') {$channel = HIDDEN;}; 
 
Thanks for your help.