HTML5 restricting input characters

Use html5 pattern attribute for inputs:

<input type="text" pattern="\d*" title="Only digits" />

OR

Use html5 number type for input :

<input type="number" />

To slightly improve off of jonhopkins excellent answer, I added backspace and delete key acceptance like so:

    function inputValidate(){
   var e = event || window.event;  
   var key = e.keyCode || e.which;                              
   if (((key>=48)&&(key<=57))||(key==8)||(key == 46)) { //allow backspace //and delete
           if (e.preventDefault) e.preventDefault(); 
           e.returnValue = false; 
   }
 }

The input textbox

<input type="text" onKeyDown="myFunction()" value="" />

JavaScript

function myFunction() {
    var e = event || window.event;  // get event object
    var key = e.keyCode || e.which; // get key cross-browser

    if (key < 48 || key > 57) { //if it is not a number ascii code
        //Prevent default action, which is inserting character
        if (e.preventDefault) e.preventDefault(); //normal browsers
        e.returnValue = false; //IE
    }
}