<input type="text" oninput="this.value = this.value.replace(/[^0-9.]/g, '').replace(/(..*?)..*/g, '$1');" />
function isNumber(evt) {
evt = (evt) ? evt : window.event;
var charCode = (evt.which) ? evt.which : evt.keyCode;
if (charCode > 31 && (charCode < 48 || charCode > 57)) {
return false;
}
return true;
}
<input type="text" class="textfield" value="" id="extra7" name="extra7" onkeypress="return isNumber(event)" />