Support optional quotes in a Boolean expression - parsing

Question

Background
I have been using ANTLRWorks (V 1.4.3) for a few days now and trying to write a simple Boolean parser. The combined lexer/parser grammar below works well for most of the requirements including support for quoted white-spaced text as operands for a Boolean expression.
Problem
I would like the grammar to work for white-spaced operands without the need of quotes.
Example
For example, expression-
"left right" AND center
should have the same parse tree even after dropping the quotes-
left right AND center.
I have been learning about backtracking, predicates etc but can't seem to find a solution.
Code
Below is the grammar I have got so far. Any feedback on the foolish mistakes is appreciated :).
Lexer/Parser Grammar
grammar boolean_expr;
options {
TokenLabelType=CommonToken;
output=AST;
ASTLabelType=CommonTree;
}
#modifier{public}
#ctorModifier{public}
#lexer::namespace{Org.CSharp.Parsers}
#parser::namespace{Org.CSharp.Parsers}
public
evaluator
: expr EOF
;
public
expr
: orexpr
;
public
orexpr
: andexpr (OR^ andexpr)*
;
public
andexpr
: notexpr (AND^ notexpr)*
;
public
notexpr
: (NOT^)? atom
;
public
atom
: word | LPAREN! expr RPAREN!
;
public
word
: QUOTED_TEXT | TEXT
;
/*
* Lexer Rules
*/
LPAREN
: '('
;
RPAREN
: ')'
;
AND
: 'AND'
;
OR
: 'OR'
;
NOT
: 'NOT'
;
WS
: ( ' ' | '\t' | '\r' | '\n') {$channel=HIDDEN;}
;
QUOTED_TEXT
: '"' (LETTER | DIGIT | ' ' | ',' | '-')+ '"'
;
TEXT
: (LETTER | DIGIT)+
;
/*
Fragment lexer rules can be used by other lexer rules, but do not return tokens by themselves
*/
fragment DIGIT
: ('0'..'9')
;
fragment LOWER
: ('a'..'z')
;
fragment UPPER
: ('A'..'Z')
;
fragment LETTER
: LOWER | UPPER
;

Answer 1

Simply let TEXT in your atom rule match once or more: TEXT+. When it matches a TEXT token more than once, you'll also want to create a custom root node for these TEXT tokens (I added an imaginary token called WORD in the grammar below).
grammar boolean_expr;
options {
output=AST;
}
tokens {
WORD;
}
evaluator
: expr EOF
;
...
word
: QUOTED_TEXT
| TEXT+ -> ^(WORD TEXT+)
;
...
Your input "left right AND center" would now be parsed as follows: