Calc.g4

grammar Calc;
eval returns[value]
	: SPACE? expr SPACE? {$value = $expr.value;}
	;
expr returns[value]
	: me1=multExpr {$value = $me1.value;}
		(SPACE? '+' SPACE? me2=multExpr {$value += $me2.value;}
		|SPACE? '-' SPACE? me2=multExpr {$value -= $me2.value;})*
	;
multExpr returns[value]
	: a1=atom {$value = $a1.value;}
		(SPACE? '*' SPACE? a2=atom {$value *= $a2.value;}
		|SPACE? '/' SPACE? a2=atom {$value /= $a2.value;})*
	;
atom returns[value]
	: INT {$value = Number.parseInt($INT.text);}
	| '(' SPACE? expr SPACE? ')' {$value = $expr.value;}
	;
SPACE: [ \t]+;
INT: [0-9]+;