[antlr-interest] modifying expression grammar ; unexpected token null

[tjs]

I am trying to modify the Expression grammar given in the 
JS Mill's tutorial (as of this time down;
http://supportweb.cs.bham.ac.uk/documentation/tuto
rials/docsystem/build/tutorials/antlr/antlrhome.html)

I have 1 main goal here:
To make it so you don't need parantheses around the top expression
I did this by recrusively defining an expression as the same as in the
tutorial (fairly straightforward), and instead of

| expr

on the atom rule, I used

| (LPAREN^ expr RPAREN!) 

Which is the way the Java grammars I've look at handle expressions.
All this grammar does is parse math expressions and enforce
parantheses (for example, 2+2 outputs 4.0, and (1-3)*4 outputs -8, or
it SHOULD anyway). Again, I modified the ORIGINAL grammar.

My PROBLEM is that when I feed it input (in the form of a text file
that is only 3 characters in length, it gives me the following error:

line 1:4 unexpected token: null
 ( + 2 )
(AST): unexpected end of subtree

My interpretation of this is that it is trying to read the 4th
character of the input (which doesn't exist, hence null) and then the
AST doesn't understand why it can't find the end of the subtree, it
apparently also thinks it needs another token when it shouldn't. I
can't figure out why it wants a 4th character or any other problem.

The following are 1. the input text 2. the grammar 3. the ORIGINAL
grammar ( which does work properly, but you need to do (2+2) instead
of 2+2, because it makes you use parantheses around everything ), 4.
the way in which I load the text file into the lexer

finally, before you think i am a "Newbie" who doesn't deserve any
help, i have read 50+ pages of ANTLR documentation, searched
everything in /docs and this forum, experimented with this grammar for
over 4 hours, looked at Java grammars to see how they handled
expressions, and wrote a very crude expression parser that was very
messy before to graph functions (its messiness is part of the reason i
am using ANTLR to redo it). i just can't figure out my dumb problem,
that's all. also, if you help me i'll give you credit in my program
for correcting my dumb mistakes!

------- 1. input text
2+2

(this is only 3 characters, no newlines, or anything else)

------- 2. my expression grammar

class ExpressionParser extends Parser;
options { buildAST=true; }

expr     :  sumExpr ;
sumExpr  : prodExpr ((PLUS^|MINUS^) prodExpr)* ;
prodExpr : powExpr ((MUL^|DIV^|MOD^) powExpr)* ;
powExpr  : atom (POW^ atom)? ;
atom     : INT | (LPAREN^ expr RPAREN!) ;

class ExpressionLexer extends Lexer;

LPAREN : '(';
FUNCSIN : 's';
RPAREN : ')';
PLUS  : '+' ;
MINUS : '-' ;
MUL   : '*' ;
DIV   : '/' ;
MOD   : '%' ;
POW   : '^' ;
SEMI  : ';' ;
protected DIGIT : '0'..'9' ;
INT   : (DIGIT)+ ;

{import java.lang.Math;}
class ExpressionTreeWalker extends TreeParser;

expr returns [double r]
  { double a,b; r=0; }

  : #(PLUS  a=expr b=expr)  { r=a+b; }
  | #(MINUS a=expr b=expr)  { r=a-b; }
  | #(MUL   a=expr b=expr)  { r=a*b; }
  | #(DIV   a=expr b=expr)  { r=a/b; }
  | #(MOD   a=expr b=expr)  { r=a%b; }
  | #(POW   a=expr b=expr)  { r=Math.pow(a,b); }
  | #(LPAREN a=expr) { r=a;}
  | i:INT { r=(double)Integer.parseInt(i.getText()); }
  ;
    
------ 3. the original grammar from Mill's tutorial (working, but noly
when the input is (2+2)  not 2+2 )

class ExpressionParser extends Parser;
options { buildAST=true; }

expr     : (LPAREN^ sumExpr RPAREN!) ;
sumExpr  : prodExpr ((PLUS^|MINUS^) prodExpr)* ;
prodExpr : powExpr ((MUL^|DIV^|MOD^) powExpr)* ;
powExpr  : atom (POW^ atom)? ;
atom     : INT | expr ;

class ExpressionLexer extends Lexer;

LPAREN : '(';
RPAREN : ')';
PLUS  : '+' ;
MINUS : '-' ;
MUL   : '*' ;
DIV   : '/' ;
MOD   : '%' ;
POW   : '^' ;
SEMI  : ';' ;
protected DIGIT : '0'..'9' ;
INT   : (DIGIT)+ ;


{import java.lang.Math;}
class ExpressionTreeWalker extends TreeParser;

expr returns [double r]
  { double a,b; r=0; }

  : #(PLUS  a=expr b=expr)  { r=a+b; }
  | #(MINUS a=expr b=expr)  { r=a-b; }
  | #(MUL   a=expr b=expr)  { r=a*b; }
  | #(DIV   a=expr b=expr)  { r=a/b; }
  | #(MOD   a=expr b=expr)  { r=a%b; }
  | #(POW   a=expr b=expr)  { r=Math.pow(a,b); }
  | #(LPAREN a=expr) { r=a;}
  | i:INT { r=(double)Integer.parseInt(i.getText()); }
  ;
 
---- 4. excerpt from my test program

ExpressionLexer lexer = new ExpressionLexer(new DataInputStream(
new FileInputStream("C:\\antlr\\EXPRINPUT.txt"))); 
ExpressionParser parser = new ExpressionParser(lexer);
    




 
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