[antlr-interest] Lexer rule with alternatives (Newbie question)

[Петров Александр]

Hello,
why such grammar doesn't work properly:
--------------------------------------------------------------------------------------
expr	:
        operand ((oper) operand)*
      	;
//Correct operands
operand	:
	INT
	;
OPER: ('+'|'-'|'*'|'/')
	;
INT	:	
	('0'..'9')+
	;
--------------------------------------------------------------------------------------
> t+1
line 0:3 mismatched input '+' expecting EOF

But if I replace the Lexer rule by Parser rule:

oper: ('+'|'-'|'*'|'/');

All work as it should. Why I should't use Lexer rule with alternatives ?

Thank you