จะกำหนดให้พิมพ์ข้อความได้เฉพาะภาษาไทยลงใน textbox ได้ยังไงครับ

<script language="JavaScript">
	function check_thai(ch){
	var len, digit;

	if(ch == " "){ 
		len=0;
	}else{
		len = ch.length;
	}
	for(var i=0 ; i<len ; i++)
	{
		digit = ch.charAt(i)
	if(!((digit >= "a" && digit <= "z") || (digit >="0" && digit <="9") || (digit >="A" && digit <="Z"))){
	; 
	}else{
		return false; 
	} 
	} 
		return true;
	}
	function checkvalue()
	{
	if(!check_thai(document.form1.name.value) || document.form1.name.value == "" )
	{
	alert('Please Enter Text Thai Only');
		return false;
	}else{return true;}
	}

	function checknumber() {
		key=event.keyCode;
		if (key<48 || key>57) 
			{
			alert("กรุณากรอกรหัสไปรษณีย์เป็นตัวเลขเท่านั้น !!");
			event.returnValue = false;
			postcode.focus();
		}
	}
</script>

<script type="text/javascript">
	function check_thai(ch){
	var len, digit;

	if(ch == " "){ 
		len=0;
	}else{
		len = ch.length;
	}
	for(var i=0 ; i<len ; i++)
	{
		digit = ch.charAt(i)
	if(!((digit >= "a" && digit <= "z") || (digit >="0" && digit <="9") || (digit >="A" && digit <="Z"))){
	; 
	}else{
		return false; 
	} 
	} 
		return true;
	}

	function checkvalue()
	{
	if(!check_thai(document.form1.name.value) || document.form1.name.value == "" )
	{
	alert('Please Enter Text Thai Only');
		return false;
	}else{return true;}
	}

	function onlynumbers(e){

    var keynum
    var keychar
    var numcheck

    if(window.event){
        keynum = e.keyCode
    }else if(e.which){
        keynum = e.which
    }
    if(keynum== 8) return String.fromCharCode(keynum);
    keychar = String.fromCharCode(keynum)
    return /\d/.test(keychar)
}
</script>