INPUT FIELD VALIDATION- LETTERS OR NUMBER

ALPHA LETTERS ONLY

$(document).on("keydown","input",function(event){
                 //alert("keydown");
                 var key = event.keyCode;
                 return ((key >= 65 && key <= 90) || key == 8);
        });

Note that the value of 8 is for the backspace key.

NUMBERS ONLY

$("input").keypress(function(e){ 
    //alert("keydown");
    if (e.which != 8 && e.which != 0 && (e.which < 48 || e.which > 57)) {         return false;     }