key Restrict in javascript

<script type="text/javascript" language="javascript">
function keyRestrict(e, validchars, casesensitives, onceevery, onceoneof) {
onceevery = onceevery ? onceevery : "";
onceoneof = onceoneof ? onceoneof : "";
if (!validchars) return true;
var key='', keychar='', obj='', i = 0;
var key = e.which ? e.which : window.event.keyCode;
var obj = e.target ? e.target : window.event.srcElement;
if (key == null) return true;
keychar = String.fromCharCode(key);
validchars = (validchars + onceevery + onceoneof);
if (!casesensitives) {keychar = keychar.toLowerCase();validchars = validchars.toLowerCase();onceevery =

onceevery.toLowerCase();
onceoneof = onceoneof.toLowerCase();}
for (i=0; i<(onceevery + onceoneof).length; i++) {
if (i < onceevery.length) {
if (obj.value.indexOf(onceevery.charAt(i)) != -1 && keychar == onceevery.charAt(i)) return false;
} else {if (obj.value.indexOf(onceoneof.charAt(i-onceevery.length)) != -1 && onceoneof.indexOf(keychar) != -1)

return false;}}
if (validchars.indexOf(keychar) != -1)
return true;
if ( key==null || key==0 || key==8 || key==9 || key==13 || key==27 )
return true;
return false;
}
</script>

Function Call in on Text Field


<asp:textbox font-size="10px" id="txt_a" maxlength="15" onkeypress="return keyRestrict(event,'abcdefghijklmnopqrstuvwxyz0123456789^')" runat="server" text=""></asp:textbox>