How to only allow numbers or text in a form

<form name="checknum" onSubmit="return checkban()">
Please enter your age:<br>
<input type="text" name="pnum">
<input type="submit" value="Submit">
</form>

<script language="JavaScript1.2">

function checknumber(){
var x=document.checknum.pnum.value
var anum=/(^\d+$)|(^\d+\.\d+$)/
if (anum.test(x))
testresult=true
else{
alert("Please input a valid number!")
testresult=false
}
return (testresult)
}

</script>
<script>
function checkban(){
if (document.layers||document.all||document.getElementById)
return checknumber()
else
return true
}
</script>