Regex that matches numeric with up to 2 decimal places

I am after a regex that will match numeric values with up to a user defined number of decimal places. Currently I have

/(^-?\d\d*\.\d*$)|(^-?\d\d*$)|(^-?\.\d\d*$)/

which will allow as many places as input but I would also like to sometimes allow 2 for currency or 4 or more for other input. The function I am building is

var isNumeric = function(val, decimals) {
    // decimals is not used yet
    var objRegExp = /(^-?\d\d*\.\d*$)|(^-?\d\d*$)|(^-?\.\d\d*$)/;
    return objRegExp.test(val);
};