docs: def instances(exp=".*"):
      "Filter list of machines matching an expression"
      expression = re.compile(exp)
      instances = []
      for node in ec2_instances():
          if node.tags and ip(node):
              try:
                  if expression.match(node.tags.get("Name")):
                      instances.append(node)
              except TypeError:
                  pass
      return instances