[sashOk]

да, немного опоздал....
Вот моё произведение:

var operators = Array(
	"+", "-", "*", "/", "^"
);

function Calc(str){
	//Раскрытие скобок:
	var pos1 = 0;
	while( pos1 || (pos1 = str.indexOf("(")) != -1){ 
		var pos2 = str.indexOf(")",++pos1);
		if(pos2>0){
			var noBrackets = str.substring(pos1,pos2);
			pos1 = str.indexOf("(",pos1);
			if(pos1 > pos2 || pos1 < 0){
				str = str.split("("+noBrackets+")").join( Calc_noBrackets(noBrackets,0) );
				pos1 = 0;
			}
		}else{
			trace("Error: No ending ')' !");
			return;
		}
	}
	return Calc_noBrackets(str,0);
}

function Calc_noBrackets(str,opId){
	str = str.split("--").join("+");
	str = str.split("+-").join("-");

	var args = str.split(operators[opId]);
	var result = 0;

	for(var argId=0;argId<args.length;argId++){
		var arg = args[argId];
		for(var nextOpId=opId+1;nextOpId<operators.length;nextOpId++)
			if(arg.indexOf(operators[nextOpId])>0){
				arg = Calc_noBrackets(arg,nextOpId);
				break;
			}

		arg = new Number(arg);
		if(argId==0)
			result = arg;
		else switch(operators[opId]){
			case "^": result  = Math.pow(result,arg); break;
			case "*": result *= arg; break;
			case "/": result /= arg; break;
			case "+": result += arg; break;
			case "-": result -= arg; break;
		}
	}
	return result;
}

Пример:

trace( Calc("34/6-(4+1)*(2+1.5)") )