Using different case keywords in ANTLR grammar - antlr

Question

Some keywords (string constant) in my grammar contain capital letters
e.g.
PREV_VALUE : 'PreviousValue';
This causes strange parsing behavior: other tokens that contain same capital letters ('P','V') are parsed incorrectly.
Here's a simplified version of the lexer grammar:
lexer grammar ExpressionLexer;
COMMA : ',';
LPAREN : '(';
RPAREN : ')';
LBRACK : '[';
RBRACK : ']';
PLUS : '+';
MINUS : '-';
MULT : '*';
DIV : '/';
PREV_VALUE : 'PreviousValue';
fragment DIGIT : ('0'..'9');
fragment LETTER : ('a'..'z'|'A'..'Z'|'_');
fragment TAB : ('\t') ;
fragment NEWLINE : ('\r'|'\n') ;
fragment SPACE : (' ') ;
When I try parsing such expression:
var expression = "P"; //Capital 'P' which included to the keyword 'PreviousValue'
var stringReader = new StringReader(expression);
var input = new ANTLRReaderStream(stringReader);
var expressionLexer = new ExpressionLexer(input);
var tokens = new CommonTokenStream(expressionLexer);
tokens._tokens collection contains one value
[0] = {[#0,1:1='<EOF>',<-1>,1:1]}
It's incorrect.
If I change expression to 'p' (lowercase letter)
tokens._tokens collection contains two values
[0] = {[#0,0:0='p',<0>,1:0]}
[1] = {[#1,1:1='<EOF>',<-1>,1:1]}
It's correct.
When string PREV_VALUE : 'PreviousValue'; is removed from grammar, both expressions are parsed correctly.
Is it possible to use different case in keywords?
Is there any example of using such keywords in ANTLR grammar?

Answer 1

I find it hard to believe a p token is created based on the grammar you posted. Lexer rules that have fragment in front of them will not produce tokens: these rules are only used by other lexer rules.
A simple demo shows this:
lexer grammar ExpressionLexer;
#lexer::members {
public static void main(String[] args) throws Exception {
ExpressionLexer lexer = new ExpressionLexer(new ANTLRStringStream(args[0]));
CommonTokenStream tokens = new CommonTokenStream(lexer);
tokens.fill(); // remove this line when using ANTLR 3.2 or an older version
System.out.println(tokens);
}
}
COMMA : ',';
LPAREN : '(';
RPAREN : ')';
LBRACK : '[';
RBRACK : ']';
PLUS : '+';
MINUS : '-';
MULT : '*';
DIV : '/';
PREV_VALUE : 'PreviousValue';
fragment DIGIT : ('0'..'9');
fragment LETTER : ('a'..'z'|'A'..'Z'|'_');
fragment TAB : ('\t') ;
fragment NEWLINE : ('\r'|'\n') ;
fragment SPACE : (' ') ;
Now generate the lexer and compile the .java source file:
java -cp antlr-3.3.jar org.antlr.Tool ExpressionLexer.g
javac -cp antlr-3.3.jar *.java
and run a few tests:
java -cp .:antlr-3.3.jar ExpressionLexer p
line 1:0 no viable alternative at character 'p'
which is correct since there is no (non-fragment) rule that starts with, or matches, a "p".
java -cp .:antlr-3.3.jar ExpressionLexer P
line 1:1 mismatched character '' expecting 'r'
which is correct since the only (non-fragment) rule that starts with a "P" expects an "r" to be the next character (which isn't there).