Any Way To Detect Certain Keys OnKeyDown?

What I need is something as such (based on previous thread): I have a textfield and I want to limit the input to only numbers, letters and caps.

So the code is:
<input onkeydown="return testChars(event)" type="text" .../>
function testChars(e){
var keyCode = e.keyCode;
if(e.shiftKey && keyCode==53){
return false;
} //block "%"
//HERE BLOCK EVERYTHING THATS NOT a Letter, caps and numbers
if ( (keyCode > 64 && keyCode < 91) ||(keyCode > 96 && keyCode < 123) )//Letters and Caps{
return true;
}else if ( keyCode > 47 && keyCode < 58 )//numbers{
return true;
}}

The problem however using the onkeydown is that I would need to cater for every single possibility of them using SHIFT+1=*, SHIFT+2, SHIFT+3 etc. which would take forever. And I cant block Shift because they might use it for capitalizing letters.