Allow only numeric input in a text input, and allow a hyphen as an optional first character

I need to automatically clean user entry into a text input - to only allow numbers, except for the first character, which could be a number or a hyphen.

So, for example, if the user types 138a29 it would be automatically updated to 13829.

I was using an onKeyPress event to check the keyCode to allow only numbers, but that's proved to be a little difficult to truly allow only numbers without breaking arrow keys, backspace keys, etc in so开发者_运维问答me browsers. And now there is a new requirement to allow a hyphen(-) as an optional first character, but a hyphen is not valid anywhere else in the string.

I have the Prototype library to use for my particular project, but no jQuery.

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The simplest way to do this is to avoid the keyCode and use the textbox's value on keyup. Something like this...

Event.observe('myInput', 'keyup', function(){
    var value = this.value, first;
    if(value.length == 0)
         return;

    first = value.charAt(0);
    value = value.replace(/[^0-9]/g,'');

    if(first == '-')
        value = first + value;

    this.value = value;
    return true;
});

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Try this

<script type="text/javascript">
function whatKey(e) {
    var thetext = document.getElementById('tt');   // Current value
    var theKey = String.fromCharCode(e.keyCode);   // key entered
    var combval = thetext.value + theKey;          // Result would be this

    return (!isNaN(combval) || combval=='-');      // if result OK, accept key

}
</script>