实现自动跳转下个输入框

function checkLen(x,y){
	if (y.length==x.maxLength){
		var next=x.tabIndex;
  		//是否到了最后一个文本框
		if (next<4){
			document.getElementById("next"+next).focus();
		}
	}
}
<input size="3" tabindex="1" id="next0" onkeypress="return event.keyCode>=48&&event.keyCode<=57" onpaste="return !clipboardData.getData('text').match(/\D/)" ondragenter="return false" style="ime-mode:Disabled" maxlength="4" onkeyup="checkLen(this,this.value)">年
<input size="1" tabindex="2" id="next1" onkeypress="return event.keyCode>=48&&event.keyCode<=57" onpaste="return !clipboardData.getData('text').match(/\D/)" ondragenter="return false" style="ime-mode:Disabled" maxlength="2" onkeyup="checkLen(this,this.value)">月&nbsp;至&nbsp;
<input size="3" tabindex="3" id="next2" onkeypress="return event.keyCode>=48&&event.keyCode<=57" onpaste="return !clipboardData.getData('text').match(/\D/)" ondragenter="return false" style="ime-mode:Disabled" maxlength="4" onkeyup="checkLen(this,this.value)">年
<input size="1" tabindex="4" id="next3" onkeypress="return event.keyCode>=48&&event.keyCode<=57" onpaste="return !clipboardData.getData('text').match(/\D/)" ondragenter="return false" style="ime-mode:Disabled" maxlength="2" onkeyup="checkLen(this,this.value)">月