Hi,

Write an onChange client script. One sample is below

function onChange(control, oldValue, newValue, isLoading, isTemplate) {

    if (isLoading || newValue === '') {

          return;

    }

  var numnericVal=g_form.getValue('your field name');

  if (isNaN(numnericVal)) // check if value is not numeric

    {

  alert("Please enter numeric values");

  g_form.setValue('your field name', ''); // Set the field to empty

    }

}