<input id=a1 onkeyup="value=value.replace(/[^\d.]/g,'') "onbeforepaste="clipboardData.setData('text',clipboardData.getData('text').replace(/[^\d.]/g,''))">
<input id=a2 onkeyup="value=value.replace(/[^\d.]/g,'') "onbeforepaste="clipboardData.setData('text',clipboardData.getData('text').replace(/[^\d.]/g,''))">
<input type=button value=getResult onClick="a3.value=parseFloat(a1.value)+parseFloat(a2.value)">
<input id=a3>
<input id=a2 onkeyup="value=value.replace(/[^\d.]/g,'') "onbeforepaste="clipboardData.setData('text',clipboardData.getData('text').replace(/[^\d.]/g,''))">
<input type=button value=getResult onClick="a3.value=parseFloat(a1.value)+parseFloat(a2.value)">
<input id=a3>
<input name=txt1>
<input name=txt2>
<input name=txt3>
<input type=button value=sum onclick="txt3.value=parseFloat(document.forms[0].txt1.value)+parseFloat(document.forms[0].txt2.value)">
</form>
<input id=a2 onkeyup="value=value.replace(/[^\d.]/g,'') "onbeforepaste="clipboardData.setData('text',clipboardData.getData('text').replace(/[^\d.]/g,''))">
<input type=button value=getResult onClick="a3.value=parseFloat(a1.value)+parseFloat(a2.value)">
<input id=a3>
<title></title>
<head>
<script language="javascript">
<!--
function IsNum(num)
{
var number="0123456789.";
for(i=0;i<num.length;i++)
{
for(j=0;j<number.length;j++)
if (num.charAt(i)==number.charAt(j)) break;
if (j>=number.length) return (false);
}
return (true);
}function getCount()
{
var val1,val2;
var theObject;
theObject = document.counttest;
val1 = theObject.text1.value;
val2 = theObject.text2.value;
if( !IsNum(val1) )
{
alert("illegal input");
theObject.text1.value = "";
return;
}
if( !IsNum(val2) )
{
alert("illegal input");
theObject.text2.value = "";
return;
}
theObject.text3.value = parseFloat(val1)+parseFloat(val2);
}//-->
</script>
</head>
<body>
<form name=counttest>
text1:<input type=text size=5 name=text1 onblur=getCount()><br>
text2:<input type=text size=5 name=text2 onblur=getCount()><br>
text1+text2:<input type=text size=5 name=text3>
</form>
</body>
</html>=============你可以再加上判断，只能有一个小数点。