Getting card number format in textbox

var str, strc;

//call this function in ontextchange
function cardNumberFormat(){
  try{
    if(str !== null && str !== undefined){
      if(frmPaymentMode.txtCardNumber.text.length > str.length){
        if(frmPaymentMode.txtCardNumber.text.length ===4){
          frmPaymentMode.txtCardNumber.text = frmPaymentMode.txtCardNumber.text+" ";
        }else if(frmPaymentMode.txtCardNumber.text.length ===9){
          frmPaymentMode.txtCardNumber.text = frmPaymentMode.txtCardNumber.text+" ";
        }else if(frmPaymentMode.txtCardNumber.text.length ===14){
          frmPaymentMode.txtCardNumber.text = frmPaymentMode.txtCardNumber.text+" ";
        }
      }else{

      }
    }
    str = frmPaymentMode.txtCardNumber.text;

  }catch(err){
    dialogue("Error "+err);
  }
}