<script>
function check() {
var reg = /^(\d{1}|\d{2}|([0-1][0-9][0-9]|[0-2][0-5][0-5]))\.(\d{1}|\d{2}|([0-1][0-9][0-9]|[0-2][0-5][0-5]))\.(\d{1}|\d{2}|([0-1][0-9][0-9]|[0-2][0-5][0-5]))\.(\d{1}|\d{2}|([0-1][0-9][0-9]|[0-2][0-5][0-5]))$/;
  if (reg.test(t1.value)){
    alert('right!');
  }
  else {
    alert('error!');
  }
}
</script><input type="text" value="127.0.0.1" id="t1">
<input type="button" value="check"  onclick="check();">