[Kolyaj]

function setValidator(id, regex) {
  var element = document.getElementById(id);
  if (element) {
    var lastValue = element.value;
    if (!regex.test(lastValue))
      lastValue = '';
    setInterval(function() {
      var value = element.value;
      if (value != lastValue) {
        if (regex.test(value))
          lastValue = value;
        else
          element.value = lastValue;
      }
    }, 10);
  }
}

<input type="text" id="test" />
<script type="text/javascript">
setValidator('test', /^[a-zA-Z_]*$/);
</script>