Negative and positive decimal validation

<script type="text/javascript">
   function IsNumFieldKeyPress() {
     var element;
     element = event.srcElement;
     if (event.keyCode >= 48 && event.keyCode <= 57) {//to handle num 0 -9 
         event.returnValue = true;
         return;
     }
     else if (event.keyCode == 46) { // to handle "."
        if (element.value.indexOf('.', 0) == -1) {
             event.returnValue = true;
             return;
        }
     }
     else if (event.keyCode == 43) { // to handle + 
        if (element.value == undefined)
            element.value = "+";
            else if (element.value.charAt(0) != "+" && element.value.charAt(0) != "-")
            element.value = "+" + element.value;
        }
        else if (event.keyCode == 45) { // to handle - 
           if (element.value == undefined)
               element.value = "-";
               else if (element.value.charAt(0) != "-" && element.value.charAt(0) != "+")
               element.value = "-" + element.value;
           }
           event.returnValue = false;
        }
</script>

^-?[0-9]\d*(\.\d+)?$

function validateNumbers() 
{ 
    var re = /^-?\d*\.?\d+$/;
var szValue = document.getElementById("txtNumber").value;
    if(re.test(szValue))
     alert("Entered numbers are allowed");
    else
     alert("Entered numbers are not allowed")
}