this will work...

var regex = /^\d*\.{0,1}\d+$/;
if (regex.test(mystring)) {
alert("good");
} else {
alert("bad");
}

however - i need the function to work on a key event so i'm going this route...

function numbers_only(e) {
var key;
var char;
var regex = /^\d*\.{0,1}\d+$/;
if (window.event) {
key = e.keyCode;
} else if (e.which) {
key = e.which;
}
char = String.fromCharCode(key);
return regex.test(char);
}

it only allows me to punch in numbers - which is beautiful, but i need to input a decimal, as well. any suggestions?