How to check the field is Numeric or Not ?

The small example shows the validation for the field is numeric or not:

<script type='text/javascript'>

function isNumeric(elem, helperMsg){

     var numericExpression = /^[0-9]+$/;

     if(elem.value.match(numericExpression)){

             return true;

     }else{

             alert(helperMsg);

             elem.focus();

             return false;

     }

}

</script>

<form>

Enter Phone No : <input type='text' id='numbers'/>

<input type='button'

     onclick="isNumeric(document.getElementById('numbers'), 'Numbers Only Please')"

     value='Check Field' />

</form>