Javascript, remove extra characters in a string

Get only numbers:

Get only LowerCase:

Get only Alpha chars:

Code:
function onlyNumbers(x){
 x.value = x.value.replace(/[^0-9]/g,'');
}

function onlyLowercase (x){
x.value = x.value.replace(/[^a-z]/g,'');
}

function onlyAlhpa (x){
x.value = x.value.replace(/[^aA-zZ]/g,'');
}