Regular expression with javascript, only numbers required

Hi experts,

Can someone tell me what the below javascript code is doing please?  I want to accept only numbers in my form fields, and it seems they accept 'x' (but no other non-numeric characters).  Ideally I'd like to accept negative numbers as well.

Here's the html:

<td align="left"><input style="text-align: center;" type="text" name="txtGames<?=$ctr?>" value="1" size="1" maxlength="1" onkeypress="return NumbersOnly(event)"/>

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<script type="text/javascript">
function NumbersOnly(e)
{
var keynum;
var keychar;
var numcheck;

if(window.event) // IE
  {
  keynum = e.keyCode;
  }
else if(e.which) // Firefox/Opera
  {
  keynum = e.which;
  }
keychar = String.fromCharCode(keynum);

numcheck = /[^\d\b\0x7F]/;
return !numcheck.test(keychar);
}

</script>

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