Regular Expression to Allow Only Numbers

Regular Expression to Allow Only Numbers

i want to give validation condition like,
var BlkAccountIdV = $('#txtBlkAccountId').val();
if (BlkAccountIdV == "") {
document.getElementById('lblAccountId').innerText = "Enter Valid
AccountID";
errorflag=1;
}
if condition should be Executed only when entered text box value contains
Letter.What value can i give inside Quotation marks(if (BlkAccountIdV ==
"") )?