input focus 자동 변경

<script>

// 포커스 자동이동 스크립트

function focus_chk(obj,tar,num){
var openerFmNm = eval("document.all."+tar+"");
	   if (obj.value.length == num){
	   	   openerFmNm.focus();
	   }else if(obj.value.length != num ){
	   	   obj.focus();
	   }
}
</script>
<table>
<tr>
<td class="td_head" height="22">전화 번호</td>
<td class="nor_w">&nbsp;
<input type="text" size="3" maxlength="3" name="ph" class="inbox" onkeyup="focus_chk(this,'ph[1]',3)">
<input type="text" size="4" maxlength="4" name="ph" class="inbox" onkeyup="focus_chk(this,'ph[2]',4)">
<input type="text" size="4" maxlength="4" name="ph" class="inbox">
</td>
</tr>
</table>

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