Limiting text input to characters and digits

To remove stuff such as punction and spaces (or anything you want) from a text box before sending to the server for validation you can use code like this, first the HTML:-

<input 
name="name" 
onblur="this.value = entrycheck(this.value)" 
type="text" 
size="20">

And then the following Javascript will remove invalid characters when the user moves to another field:-

<script type="text/javascript">
function namecheck(theInput) {
  var valid = 'abcdefghijklmnopqrstuvwxyz1234567890';
  var test ='';
  var ret ='';
  for(i=0;i<theInput.length;i++) {
    test = theInput.substr(i,1);
    if(valid.indexOf(test.toLowerCase()) != -1) {
      ret = ret + test;
    }
  }
  return ret;
}
</script>

In this example I just want digits and characters. If you want to add certain punctuation such as stops and commas just add them to the valid variable.