On the homepage of www.antlr.org you can see this sample grammar:
grammar Expr;
prog: (expr NEWLINE)* ;
expr: expr ('*'|'/') expr
| expr ('+'|'-') expr
| INT
| '(' expr ')' ;
A little editing and it will be what you need. This is for ANTLR 4. Which version are you using? I'm sure every version of ANTLR has an expression grammar sample.