Assign null value to parameter in jquery function

How to Assign null value to parameter in this function

<script type="text/javascript">
$(document).ready(function(){
function alpha_numeric(an) {
var TCode = document.getElementById(an).value;
for(var i=0; i<TCode.length; i++)
{
var char1 = TCode.charAt(i);
var cc = char1.charCodeAt(0);
if((cc>47 && cc<58) || (cc>64 && cc<91) || (cc>96 && cc<123) || (cc==32) || (cc==95))
{
}
else
{
alert('Special character are not allowed\n\nPlease enter only alphanumeric and underscore');
//here i want to assign null value parameter or input field(#filcompname) but field name or id is not static it's dynamic or multiple field
return false;
}
}
}
$('.wrapper .accountinfo .information li.inputs #filcompname').live('keyup',function(){
var an = $(this).attr("id");
alpha_numeric(an);
});
});
</script>

Code (markup):