calculate.js

function calc(expression) {
    var divmultest = /[\/\*]+/;
    var divmulreplace = /(\-?[\d\.]+)\s*(\/|\*)\s*(\-?[\d\.]+)/g;
    var addsubtest = /\d+\s*[\+\-]+/;
    var addsubreplace = /(\-?[\d\.]+)\s*(\+|\-)\s*(\-?[\d\.]+)/g;
    var parens = /\(([^\(\)]+)\)/g;

    var evaluate = function (e) {
        while (divmultest.test(e)) {
            e = e.replace(divmulreplace, function (m, x, o, y) {
                return o == '*' ? x * y : x / y;
            });
        }
        while (addsubtest.test(e)) {
            e = e.replace(addsubreplace, function (m, x, o, y) {
                return o == '+' ? x * 1 + y * 1 : x - y;
            });
        }
        return e;
    }

    while (expression.indexOf('(') !== -1) {
        expression = expression.replace(parens, function (m, e) {
            return evaluate(e);
        });
    }

    return +evaluate(expression);
}



calc('123.45*(678.90 / (-2.5+ 11.5)-(80 -19) *33.25) / 20 + 11'); // => -12042.760875