Javascript : Enter Maximum value, minimum value and certain variables in textbox

hey all,

Below are the javascript to check on the textbox value. The textbox only allow user to enter 0 and the maximum value 100 but I also need to allow the user to enter '-' and 'X'. How do I modify the code to suits the criteria?

<input type="text" class="text-input small-input" maxlength="3" size="1" name="tov[]" value="0" onkeyup="this.value = minmax(this.value, 0, 100)"/> 


<script type="text/javascript"> 
function minmax(value, min, max) 
{
    if(parseInt(value) < 0 || isNaN(value)) 
        return 0; 
    else if(parseInt(value) > 100) 
        return 100; 
	  else return value;
}
</script>

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Thanks.